Here is a simple tip.
If you want people to join your community, don’t ask them to join. Ask them if they want to join.
e.g. Don’t publish a statement to your mailing list or social media audience similar to: “Join our new online community at www.communitylink.com”.
Instead, publish a message asking “Would you like to join a group/community which {activity}”
There is some psychology behind this. The former gets associated with another selfish marketing effort. The latter solicits an initial commitment which provokes members to join. The latter is friendlier too.
Now work through those that have committed and give them something specific they can do in the community (after they have signed in, naturally)
Engage your members first. Not only does the second approach get you more members, it gets you more active members. You also wont bother the uninterested members with requests to join.
You can also use this for things you want members to do within the community.