There is an online stoic community that even has a map with the locations of members.
For those not deeply read on the subject, Stoicism is relevant because the belief system roughly holds (1) that the only unqualified good thing is personal virtue, (2) that most human mental states (like emotions and preferences in addition to declarative cognitive content) are in some sense states of belief, and (3) that virtue consists in having certain and true beliefs (broadly construed) about what is good. I'm emphasizing points that might interest LWers (for example Stoics had an interesting angle on akrasia) and leaving out details for succinctness.
I've been playing with the idea of sending invitations to some people on the New Stoa map when I know about LW meetups happening near them that would be friendly to their attendance. I've never taken steps on this thought and have no data on the feasibility or value of this effort, but it seems worth putting here in case someone else wanted to try it :-)
At Balticon, there was a table for humanists, the Ethical Society, and skeptics. I told one of the folks there about Less Wrong-- he seemed interested and I'll check back with him, but he'd never heard of it at all-- it took a little time to disambiguate between Less Wrong and what he heard as a human being named Les Wrong.
Anyway, what do you think of doing outreach to fairly compatible groups (I'd add Unitarians to the list)? And if so, what would be good methods?