Here on TMC? Interpreting your question literally...probably not without a strong reason, because logically we'd end up having to make a bunch of forums for all the different variants of the Model S (and other models too).
A little more generally...note that anybody can start a thread (like this one you just started!)...would that work for you?
Also I've seen "groups" on TMC for uncommon Model S variants (most notably the owners of the rare Autopilot P85s). So if you wanted to be able to discuss topics specific to the S90, you might be able to ask the admins for that. (On the other hand I once joined a group here on TMC for dual-motor cars, back when that was new, and very little came out of that group.)
My curiosity is getting the better of me...is there something particularly special about the S90 that I'm not aware of? (S85D owner here.)
Bruce.
PS. I see that was your first post...welcome to TMC!