I've been asked a few times in the last week or so if I had any ideas for increasing membership in an established online community.  After giving it some thought, I decided it might be fun to brainstorm here.  Below are five ideas that can take your online community to new heights...

1. Are you talking to your offline community? If you have a real-world presence, are you making those folks aware of your online community? It could be as simple as a sign at your point of sale, an article in your magazine, or a tag attached to items you're selling. Do you have the URL of your community on your business cards? In your email signature? Inside your conference materials and presentations?
2. Have you asked your members to invite their friends? People who are passionate about your community already may not have thought to include their real-life circle of friends.  By making an explicit request, you may be sparking someone to make the effort and invite more people to the party. You could even include a reward (like Karma points or special avatars) for those who follow through.
3. If you are posting "trigger" topics or recurring discussions, have you tied them to your overall SEO keywords? Do you have keywords that you're targeting? If so incorporating them into the structure of your online community will help with discovery.  Remember that unless you've blocked search engines from crawling your community, all pages are potential content that can show up in search results, and thus entice new members to register.
4. How about mining Twitter for interested members? Why not create a Twitter account for your online community and then tweet relevant content from existing members?  You can magnify your impact by using hashtags to catch interested eyeballs.
5. Do you share content from your online community in other places? You can draw attention to your community by incorporating fresh content from it on other pages of your website, within the pages of your newsletter or magazine, in any ads you are already running, in your tweets, etc. Shine the spotlight on your content every chance you get, and you can draw new members from unexpected places!

If you have ideas you'd like to share, please post here so that we can discuss further.