Funny, we just finished getting the bugs out of a trigger as well that automatically builds an opportunity team on the opportunity based on a lookup on the opportunity called "Team", which references a custom object I made called "Team" with a child object called "Team Members" where you can create all the team members on your team (each team member record references a user) and that way you can set your team on the lead, and once the opportunity is made, it automatically builds all the members who are a part of your team and sets the access level defined in their Team Member record, along with their default role, etc.

Some advice I should probably give you based on a lot of frustration, is that by default, any Opportunity Team Member records you create will have Read-Only permissions.  If you wish to give team members Read/Write permissions, then you need to insert a record in to the OpportunityShare object with an access level of "Edit" for that user.  Also, if the opportunity owner also happens to be one of the OpportunityTeamMembers that are getting added, you need to leave out the OpportunityShare record that gets created for that team member or else the trigger will error (because salesforce adds an OpportunityShare record automatically for the opportunity owner, which will conflict with the OpportunityShare record that the trigger tries to create if the Opportunity Owner is also one of the Team Members).  It was kind of tricky but eventaully we got all the bugs out.

I don't know if it's of any help, but here's the final trigger we made:

trigger OpportunityTeamMembersAssignment on Opportunity (after insert, after update) {

	if (TriggerFlags.allowOpportunityDML) {
		//Opportunity->Team
		Map<Id, Id> MOpportunityTeam = new Map<Id, Id>();
		Map<Id, Id> MOpportunityOwners = new MAp<Id, Id>();
		
		for (Opportunity o : [
								SELECT Id, Team__c, OwnerId 
								FROM Opportunity
								WHERE Id IN: Trigger.newMap.keySet()
							  ]) {
								MOpportunityTeam.put(o.Id, o.Team__c);
								MOpportunityOwners.put(o.Id, o.OwnerId);
		}
		
		//Team->UserIDs													
		Map<Id, List<Team_Member__c>> MTeamTeamMembers = new Map<Id, List<Team_Member__c>>();
		
		//Collect Team Members' User Ids
		for (Team_Member__c tm : [	SELECT Team__c, User__c, Default_Role__c, Share_Permission__c, Active__c 
									FROM Team_Member__c
									WHERE Team__c IN: MOpportunityTeam.values() 
								  ]) {
								  
								  //get set of team's users
								  List<Team_Member__c> teamUsers = MTeamTeamMembers.get(tm.Team__c) != null ?
								  										MTeamTeamMembers.get(tm.Team__c) :
								  										new List<Team_Member__c>();
								  //add team member to team
								  teamUsers.add(tm);
								  
								  //update team map
								  MTeamTeamMembers.put(tm.Team__c, teamUsers);
		}
		
		List<OpportunityTeamMember> OTMs 	= new List<OpportunityTeamMember>();
		List<OpportunityShare> OSs			= new List<OpportunityShare>();
		 
		//Create OpportunityTeamMemberRecords
		for (Id oId : MOpportunityTeam.keySet() ) {			
			//get team
			Id teamId 					= MOpportunityTeam.get(oId);
			
			//get team's team_members
			List<Team_Member__c> TMs 	= MTeamTeamMembers.get(teamId) != null ?
											MTeamTeamMembers.get(teamId) :
											new List<Team_Member__c>();
											
			for (Team_Member__c tm : TMs) {
				//create opportunity team member
				OpportunityTeamMember otm 	= new OpportunityTeamMember();
				otm.OpportunityId 			= oId;
				otm.UserId					= tm.User__c;
				otm.TeamMemberRole			= tm.Default_Role__c;			
				OTMs.add(otm); 			
				
				//create opportunity share
				OpportunityShare os 		= new OpportunityShare();
				os.OpportunityId			= oId;
				os.UserOrGroupId			= tm.User__c;
				os.OpportunityAccessLevel	= tm.Share_Permission__c;
				if (MOpportunityOwners.get(oId) != os.UserOrGroupId ) 
					OSs.add(os);
			}
			
		}
		
		//DML
		insert OTMs;
		insert OSs;
	}	
}

Test Class:

public with sharing class TestTriggerOpportunityTeamMembers {

	public static testmethod void test_1() {
		if (TestUtility.isActiveTrigger('OpportunityTeamMembersAssignment')) {
			//create test users
			User john 	= TestUtility.createTestUser();
			User paul 	= TestUtility.createTestUser();
			User george = TestUtility.createTestUser();
			User ringo 	= TestUtility.createTestUser();
			
			//create teams
			Team__c beatles 	= new Team__c();
			beatles.Name 		= 'The Cloud Beatles';
			insert beatles;
			
			Team__c wings 		= new Team__c();
			wings.name			= 'Wings';
			insert wings;
			
			//create team members		
				//john
				Team_Member__c tm1	= new Team_Member__c();
				tm1.Team__c 		= beatles.Id;
				tm1.User__c			= john.Id;
				tm1.Default_Role__c	= 'Marketing Director';
				tm1.Access__c		= 'Read/Write';
				insert tm1;
				
				//paul
				Team_Member__c tm2	= new Team_Member__c();
				tm2.Team__c 		= beatles.Id;
				tm2.User__c			= paul.Id;
				tm2.Default_Role__c	= 'Marketing Consultant';
				tm2.Access__c		= 'Read/Write';
				insert tm2;
				
				//george
				Team_Member__c tm3	= new Team_Member__c();
				tm3.Team__c 		= beatles.Id;
				tm3.User__c			= george.Id;
				tm3.Default_Role__c	= 'Marketing Consultant';
				tm3.Access__c		= 'Read/Write';
				insert tm3;
				
				//ringo
				Team_Member__c tm4	= new Team_Member__c();
				tm4.Team__c 		= beatles.Id;
				tm4.User__c			= ringo.Id;
				tm4.Default_Role__c	= 'Marketing Consultant';
				tm4.Access__c		= 'Read/Write';
				insert tm4;
			
				//paul and linda
				Team_Member__c tm5	= new Team_Member__c();
				tm5.Team__c 		= wings.Id;
				tm5.User__c			= paul.Id;
				tm5.Default_Role__c	= 'Marketing Consultant';
				tm5.Access__c		= 'Read/Write';
				insert tm5;
			
			//create test account
			Account a 	= new Account();
			a.name		= 'Teston';
			insert a;
			
			//create test opportunity	
			Opportunity o 	= new Opportunity();
			o.Name			= 'Test Oppty';
			o.AccountId		= a.Id;
			o.CloseDate		= Date.today();
			o.StageName		= 'Offer';
			o.Team__c		= beatles.Id;
			o.OwnerId		= john.Id;		
			insert o;
			
			
			List<OpportunityTeamMember> OTMs = [SELECT id, UserId, OPPORTUNITYACCESSLEVEL
												FROM OpportunityTeamMember
												WHERE OpportunityId =: o.Id
												];
		
			Set<Id> BeatlesUserIDs = new Set<Id>{ 	john.Id, 
													paul.Id,
													george.Id,
													ringo.Id
												 };
												 
			system.assertEquals(4, OTMs.size() );
			system.assert( BeatlesUserIDs.contains( OTMs[0].UserId ) );
			system.assert( BeatlesUserIDs.contains( OTMs[1].UserId ) );
			system.assert( BeatlesUserIDs.contains( OTMs[2].UserId ) );
			system.assert( BeatlesUserIDs.contains( OTMs[3].UserId ) );

		
			//update the opportunity
			User peter = TestUtility.createTestUser();
			
			//add the 5th beatle to list
			BeatlesUserIDs.add(peter.Id);
			
			//add the 5th beatle team member
			Team_Member__c tm6	= new Team_Member__c();
			tm6.Team__c 		= beatles.Id;
			tm6.User__c			= peter.Id;
			tm6.Default_Role__c	= 'Marketing Director';
			tm6.Access__c		= 'Read/Write';
			insert tm6;
			
			//update O to execute the trigger
			update o;
			
			//re-query
			OTMs = [SELECT id, UserId, OPPORTUNITYACCESSLEVEL
					FROM OpportunityTeamMember
					WHERE OpportunityId =: o.Id
					];
			
			//new test
			system.assertEquals(5, OTMs.size() );
			system.assert( BeatlesUserIDs.contains( OTMs[0].UserId ) );
			system.assert( BeatlesUserIDs.contains( OTMs[1].UserId ) );
			system.assert( BeatlesUserIDs.contains( OTMs[2].UserId ) );
			system.assert( BeatlesUserIDs.contains( OTMs[3].UserId ) );
			system.assert( BeatlesUserIDs.contains( OTMs[4].UserId ) );

					
		}
	}
	
}