From Yuku Help

Joining an open Community

Most communities are open to the public, and joining is as simple as making your first post. Once you post, you will be added to the membership list.

Joining a private Community

If a community is private, you may be requested to apply for membership first. If this is the case, you'll be given an application form to use when you try and access the community. You need to be logged in to apply to join a community.