Close, the first should work. Try this, modified a bit for clarity:
Tracy


public function getTeamList():String {
var aSelectedTeams:Array = frmProjectTeam.selectedItems;
var teamMembers:String = ""; //need to init this or the first += will error
for (var i:Number = 0; i < aSelectedTeams.length; i++) {
teamMembers += aSelectedTeams[i].uniqname;
}
return teamMembers;
}