#cephan - As far as I know, you cannot write a trigger against the AccountTeamMember object. I had the same issue and to get around it, I created a Text field on the object I needed and used an Apex Trigger to grab the information I needed from the AccountTeamMember object. Now, the only problem with this is that I haven't figured out a way to update the field if the Account team member changes. The only way around this is to literally update the record / object, which will re-import the information.

I'm not the best coder, and this is problem is somewhat knew to me, so I'm sure someone else here has a much better idea and / or code. Also, you have to watch out for no Account Team Members, as it will throw a QueryException.  Also, I am specifically looking for a certain team role in this code, however, there are times when there are two of the same type, so that's why I limit the return result to 1. I have a much bigger code block for dealing with other issues, but this is just a small snippet to help you through your problem.

trigger MyObjectUpdates on MyObject (before update, after insert) {
    for (MyObject myObj : Trigger.new){
        try{
            Id samID = [select UserId from AccountTeamMember where AccountId = : myObj.Account_link_id and TeamMemberRole = 'Strategic Account Manager' limit 1].UserId;
            string userName = [select Name from User where Id = :samID].Name;
            myObj.Account_Manager__c = userName;
        }
        catch(QueryException qEx){
            //Do something if no AccountTeamMember; We chose to send an email to the Account Managers to notify them of the missing data
        }
    }
}

I hope this helps. And, if anyone else has a better route to take, I would love to hear it as the updating is becoming an issue.

Thank you,

Wes Cooper