Ok. you shouldn't have to use the Ajax Toolkit to accomplish this task.  What you are going after sounds sort of like the "Add to Campaign" button you'll find on the contact list page ("/003" ) .

I've cooked up a brief example that shows how you might mimic that with a custom "Account_Plan__c" object where the junction between that and Contact is "Account_Team_Member__c".  Hopefully you (and anyone reading this later can adjust this example to their needs).

After you create a page/controller extension per below then you can create a list button for Contact which can then be added to the Contacts list view page.

Here you go....

Page:

<apex:page standardController="Contact" recordSetVar="Contacts" extensions="contactSetExt">
<apex:form>
<apex:pageBlock>
<apex:pageMessages/>
<apex:pageBlockButtons>
<apex:commandButton value="Go" action="{!createMembers}"/>
</apex:pageBlockButtons>
<apex:pageBlockSection>
<apex:pageBlockSectionItem>
<apex:outputLabel value="Account Plan" for="plan"/>
<apex:outputPanel>
<apex:selectList value="{!planId}" size="1">
<apex:selectOptions value="{!planOptions}"/>
</apex:selectList>
</apex:outputPanel>
</apex:pageBlockSectionItem>
</apex:pageBlockSection>
<apex:pageBlockSection title="Selected Contacts" columns="1">
<apex:pageBlockTable value="{!selected}" var="c">
<apex:column value="{!c.name}"/>
</apex:pageBlockTable>
</apex:pageBlockSection>
</apex:pageBlock>
</apex:form>
</apex:page>

 Extension Class:

public class contactSetExt {

List<Contact> contacts;
public contactSetExt(ApexPages.StandardSetController controller) {
contacts = (List<Contact>) controller.getSelected();
}

public Id planId { get; set; }

public List<ApexPages.SelectOption> getPlanOptions() {
List<ApexPages.SelectOption> options = new List<ApexPages.SelectOption>();
options.add(new ApexPages.SelectOption('','--SELECT--'));

for(Account_Plan__c p:[select name from account_plan__c order by name asc]) {
options.add(new ApexPages.SelectOption(p.id, p.name));
}

return options;
}

public PageReference createMembers() {
/* see if there are existing members for this plan */
Set<Id> existingMemberContactIds = new Set<Id>{};

for(Account_Team_Member__c m:[select contact__c from account_team_member__c where account_plan__c = :planId and contact__c in :contacts]) {
existingMemberContactIds.add(m.contact__c);
}

List<Account_Team_Member__c> newMembers = new List<Account_Team_Member__c>();

for(Contact c:contacts) {
if(!existingMemberContactIds.contains(c.id)) {
newMembers.add(new Account_Team_Member__c(account_plan__c = planId, contact__c = c.id));
}
}

PageReference p;

if(newMembers.size() > 0) {

try {
Database.insert(newMembers);
p = new ApexPages.StandardController(new Account_Plan__c(id = planId)).view();
} catch (System.DMLException e) {
ApexPages.addMessages(e);
}

}

return p;
}

}