Set creator's role based on a field value

My use case:

I'm using a group for two purposes - to allow individuals / organizations doing in-person trainings to list their events, post contact info, etc. (owners) and for their former students to leave reviews of the class / trainer / org / etc. (reviewers).

The issue is that both these categories of people (owners & reviewers) need to be able to create a group, but their default role needs to be different. I'll know if the creator is a reviewer or owner creating a field on the group that asks the question.

The current plan is to set permissions on the "member" role to be appropriate for reviewers, and those on "administrator" to be appropriate for owners.

If a reviewer creates a group, they'll get "member" permissions automatically so there's no problem there. However, I cannot figure out how to assign a role ("administrator" in this case) to the user who just created a group.

My attempts so far:

It looks like this should be possible via Rules, but I've spent much of the last day and a half trying every combination of events and actions I thought seemed reasonable with no luck. I've also searched the issue queue for references to rules and read what I could find that looked relevant, again with no luck.

If anyone can point me in the right direction for how to use Rules to make this happen, that'd be great. I'd be happy to write up some documentation once I understand how to do it and have created an example or two if that'd be helpful.

Also, if there's a different / more appropriate way to make this happen that doesn't involve Rules I'd be fine with that too. I'm not wedded to Rules, it's just what seemed to make most sense when I started on this.

Thanks in advance for any help or advice!

DanG