Allowing someone to quickly and easily get the attention of a group of users is faster and more reliable than trying to mention each member of the group individually. For example, if I wanted all of my customer support people to look at something, I'd want to use something like @Support or @Updaters.

If you do groups of users as opposed to all those at a certain user level, I'm not sure if this would require an additional field for the users to indicate the group they're a part of. If you went with user levels, it should probably look at their level on the current project, rather than their overall user level.