It's necessary to go via /admin -> Group Types -> Groups -> Add a group of this type. Adding members is confusing too (Members / Administration / Add Member ). And make sure to add members of type "Membership", not "User profile" if your installation still offers that - that was a bug fixed by Don recently IIRC.
Then you can grant permissions to that group on the bug-tracker instance via the site-map, or maybe there is a special way in the bug-tracker admin area for that.
No idea if that is the correct way - at least it worked for me a few times.
Yes, the UI is horrible.
