Hmm, it looks like you’re mod of ~three fairly dormant communities that have very small user bases. Unfortunately, at that size I wouldn’t think there’d be much in the way of regular comments, much less guest posts.
In my case I was lucky, because a co-mod and regular poster happened to join in early-on, and we were able to build up the first couple hundred users fairly quickly.
But something else that I think helped a lot was that our community is very visual-oriented, so it was pretty easy to find users who were perfectly happy to join up just to look at pretty images without necessarily clicking links or putting too much thought in to anything deeper. So pandering to the lowest common denominator of user interest seems to work nicely for building up base numbers. That said, there’s still a lot of growth we need to do, which likely involves outreach of some kind or another.