If you go to the project's page, you can leave a message there, or you can send a private message to the project leader or it's coordinator. Some projects are even more "self-service." You can simply start following the project's tag and add your own name to the list of members.
To find projects, just look in the upper right hand corner on any WikiTree page under FIND and PROJECTS.
If you need any more help, just ask!
