A given group should be able to promote another group or public instance. A user story for this one is a performer's group promoting the instance where they're performing, even if it's not a group they own.

I think it would look like any other group instance, but with something to indicate what group it is (ideally including its name and icon), or if it's a non-group instance (public).

I don't think this would be very useful outside of public and group instances? For group+ instances, it might consider members of the promoting group as "friends" who are able to join the group instance, as long as the person who promoted it is a member of that group. For regular "group" instances, you'd have to be taken to the group page first to join or request to join it.