This feature was asked for previously, but for a different reason, and was closed as not being necessary.
I believe it would be extremely useful to have a concept of a group of users purely for the purposes of simplifying project membership. If there are even a modest number of users, combined with a modest number of projects, the process of managing project membership starts to become exponential. Since it is often the case that a new user should get the same access rights as an existing user, it becomes greatly simplified if it is possible to just add that user to the appropriate group, where the group itself is a member of the project. A typical use-case is where you might have a group corresponding to a set of people at a particular customer, and another group representing your developers.
Of course it does mean that the code for presenting list boxes of users for issue assignment is slightly more complex, but not incredibly so.
An additional idea would be to allow the issue to be assigned to either an individual or a group. I think this might possibly intersect with feature request #525
We are happy to work on a patch for this...