There used to be an Ann Arbor LW meetup group, actually, back when I lived there—it seems to be pretty dead now best I can tell but the mailing list still exists. It’s A4R-A2@googlegroups.com; I don’t know how relevant this is to you, since you’re trying to start a UM group and many of the people on that list will likely not be UM-affiliated, but you can at least try recruiting from there (or just restarting it if you’re not necessarily trying to specifically start a UM group). It also used to have a website, though I can’t find it at the moment, and I doubt it would be that helpful anyway.
According to the meetup group list on this website, there’s also is or was a UM EA group, but there’s not really any information about it? And there’s this SSC meetup group listed there too, which has more recent activity possibly? No idea who’s in that, I don’t know this Sam Rossini, but possibly also worth recruiting from?
So, uh, yeah, that’s my attempt (as someone who hasn’t lived in Ann Arbor for two years) to survey the prior work in this area. :P Someone who’s actually still there could likely say more...
That’s a helpful place to start, thank you!