The boundary is what separates members from non-members.
The boundary isn't the registration form. It's the experiences, skills, relationships, accomplishments and interests that people have gained or acquired to be an accepted member of the community.
Last week I spoke at the Meet-Up Organizers group. This group of individuals each run a meet-up group. Running a successful meet-up, and all the work that goes into running a meet-up, is a boundary that individuals have to cross to become an accepted member of the group.
The tougher the boundary the greater the sense of similarity between members and the stronger the sense of community. Numerous studies link strong sense of similarity to higher levels of participation in a community.
But tougher boundaries mean less members. Most organizations keep their boundaries (the specific topic matter they're building their community around) weak to attract more members. This is a mistake.
You should keep boundaries high. Don't build a community for those that ride your motorcycles (or use your products), build a community for motorcycle riders who have travelled from coast to coast, or can fix any problem on a bike, or are based in a specific location, or are of a certain age or in certain professions, or have been customers for decades.
It's easier to build and sustain a community that has a tough boundary than one with a weak boundary.