While editing roles, there is a checkbox called Issues can be assigned to this role.
Would it be easy to have the same sort of checkbox for a group ?

h3. Context

Since Issues can now be assigned to groups, we would like to use those groups to affect issues to an entire Team (today we use special users that represent the team). But we also have "helper" groups, that only serve to easily handle rights on projects, and we would not like to see those as potential assignees. So the easy fix would be to set some option on those groups to prevent them from appearing in the assignee list box.