I came across a handy Linq operator called the Except operator today that fulfilled a simple requirement I had to only allow the selection of an item for adding to a group if that item was not already in the group. It worked great and everyone is happy with the end result.

For more info on the Except operator checkout:

• http://msdn.microsoft.com/en-us/vcsharp/aa336761.aspx#except1
• http://www.hookedonlinq.com/ExceptOperator.ashx