Set<Id> userIds = new Set<Id>();
for (GroupMember curr: IstGroupMember)
{
userIds.add(curr.UserOrGroupId);
}
List<User> theUsers = [SELECT Id FROM User WHERE Id IN: userIds];
Set<Id> userIds = new Set<Id>();
for (GroupMember curr: IstGroupMember)
{
userIds.add(curr.UserOrGroupId);
}
List<User> theUsers = [SELECT Id FROM User WHERE Id IN: userIds];
All Answers
You can consolidate this from 3 queries to 2, actually:
List<GroupMember> lstGroupMember = [Select UserOrGroupId From GroupMember Where Group.Type='Queue' and GroupId=:groupId];
and then use Damien__c's logic to build the list of user ids to query once more.