To increase membership, one can:

• Advertise on meetup.com
• Put up fliers on a university campus
• Invite your extramural friends

If your group has a decent number of members but too few core members, this means that most of your membership isn't getting value out of the community:

• Your meetup content might not be varied enough (see section on meetup content)

Based on the suggestions offered here so far, version 4 of the document (not yet public) contains the following:

How to attract new members

Sooner or later, you may wish to find new members. If you have been posting meetup notices on Less Wrong, any regular readers with an interest in showing up have probably already done so. Still, there may be readers who have seen the meetup notices or who might even be subscribed to your mailing list, but haven’t been motivated enough to attend. You can try to make them more interested by documenting how much fun you’re having: write down what you’ve done at various meetups and how people have found those meetups. If the reports sound fun enough, others will be interested in showing up. If the reports don’t sound fun enough, your first priority is making the actual meetups more interesting.

Eventually, though, you may need to reach out to people who aren’t already Less Wrong readers. Getting outsiders interested requires framing the purpose of the group appropriately. You could call yourself a philosophy or psychology discussion group, a self-improvement group, a group of people interested in efficient altruism, or whatever best fits the actual goals of your group. Be sure to come up with a description that actually matches the function of your group. If you say that your group is about self-improvement when it’s actually about discussing philosophy, people who come looking for self-improvement will leave disappointed, while people who would have been interested in philosophy never visit in the first place.

Once you have a description of the group that seems interesting to outsiders, it’s time to advertise it:

• If you have friends who might be interested in the topics discussed at the meetups, ask them to attend sometime.
• Post the reports of fun meetups where people can see them. If the person writing them has a personal blog, they could be posted there. Others can then share a link to the report on Facebook, Twitter or similar, mentioning that they attended the meetup in question and it was fun.
• Print out fliers advertising your meetup group and put them up in places that have lots of people with intellectual interests. University campuses might be good.
• See if there are any local skeptic, science, philosophy, etc. groups that you could advertise to. If at a university, try suitable student societies.
• Create a group on meetup.com (costs $12/month).

If the content of your meetups is interesting and fun enough, the new people who visit once will choose to come again. The main difference between planning a meetup for Less Wrong regulars and planning it for people who don’t know of the site is that non-readers will have less knowledge of the Less Wrong jargon and concepts. So choose activities that don’t require a deep understanding of such things, and try to avoid the jargon if you can.

Comments?

To start it, make one big meetup that you think will draw people. Make it at an exciting place in the area, chose an interesting meetup topic, or plan a fun activity. An alternative if you're really desperate for people is to draw on the surrounding area by posting in more public places.

From there, all that has to happen is a successful Less Wrong meetup, using the strategies in the OP. That is usually enough to get some of the members to come back a few times, and if the meetups continue being interesting, new members will become regulars.