You have to literally create a profile, make is as identical to the SAB as possible (mostly this means name, service area, website, phone number, don't bother with other elements) and then trigger a merger via suggestions. Then you will be controlling a duplicate (or maybe the profile directly, if you're really lucky!) and have the possibility of asking for access on the target SAB. And if you're not granted access, you'll have to ALSO verify the target SAB.
It is a very awkward and stupid procedure.
And on top of that, merging seems to have a high likelihood of triggering the SAB ranking bug...