I agree with the other answers that there are lots of online communities for community managers, many of which I'm only learning about now! I do follow the Community Roundtable though on Twitter, and I'll be following some of the other communities mentioned here as well.
That said, I should mention two communities here that haven't been mentioned yet.
The first that I would recommend is Evan Hamilton's Community Manager Breakfast. Originally a Meetup group, it's now a mailing list that also hosts monthly morning meetups ("breakfasts") in San Francisco. I generally find the information shared on the list to be top-notch, and it is my sincere hope at this point that I'll actually get to attend a Community Manager Breakfast the next time I'm in San Francisco.
The second one though is more for community managers in open-source communities, but it should work great as well. That would be the Community Leadership Forum, run by the folks behind the O'Reilly Media-sponsored Community Leadership Summit held right before OSCON. I'm not too active on the forums, but the insights shared there are great. :)
If it helps, community managers also follow growth hacking groups, since growth hacking is often required for successful community management.