is that we can't get too many new people involved at once, or it would change the dynamic so that we'd no longer be an explicitly LW group

This has worried me as well, and is why we havn't been fishing randoms from everywhere.

gender ratio

The only female attending ours is my wife. I guess it varies a lot.

extroverts

hmm. we have no extroverts and have regular awkward moments and rough flow sometimes, but we've all agreed that awkward silences don't have to be considered awkward. That has helped some.