Apparently I hit my 750-friend limit. Then I joined a community and checked the "add community to friends list" box... and it did. "Friends: 753: View Friends." When I try to use add.bml to modify which filters the community is on, it gives me an error about having too many friends, but doesn't seem to remove the community from my friends list, and I can still move it around using editgroups.bml.
Time to clean up my friends list, I guess! In the meantime, thought this might offer interesting things to poke at.