Project admins can set rights for users groups, but they can't know who is in a group.
Only server admin can view the list of users of a group.
In my large organization, project admins are in touch with users, but a project admin can't answer a user who said "why can't I do this to your project ?" when this user is supposed to be in a group.

Should it be possible to add this feature in BitBucket ?
This feature could be implemented in the project roles configuration, maybe a popup window which lists users when the cursor is on a group.
This feature could be enabled/disabled via global rights administration.
Thanks in advance !