The more contact you have with members, the more involved they will be.
If you were building a community in the real world, you would probably contact members often. You would check in on them and make house calls. You would spend time persuading them to get involved. This used to be the only way to develop a community.
Online communities don’t do this because they don’t have to. You can have a community without high levels of contact. Most online communities take the easy low-contact path. This is a shame. It means they get low involvement (see 90-9-1).
If you want high involvement, you need high level of unique contact with members. You should contact members often (with personal messages). You should constantly persuade members to be more involved. You should point out activities they can participate in. You should build real relationships with members.
The harder you work at this, the more successful you will be.