Unfortunately you cannot use a collection and do what you want to do. LANDesk developers don't actually create the data relationship from the new instance of the collection to the incident until after you have saved it, which is too late. So the trick is to keep your 'current user in group' as an attribute on the incident object itself and use a windowless action instead. The filter on your incident attribute would be something like Groups.Group = CurrentAssignment/Group That will then just display the list of users within the group the incident is currently assigned to. You can then use your assignment action to refer to this incident attribute as the user on the assignment.
The main downside of this trick is that the contents of this field isn't cleared when you use the action again. So you have to teach people to clear it first (or write some sort of very dodgy trigger). This has been an outstanding and very common bump in the road for lots of people, so welcome to the club!
Thanks it definately is a bump in the road