As a community owner, you can reach out to people who you think might be interested in your community by inviting them to become members. Members of a public community can also invite people to join the community. The invitees receive an email inviting them to join the community, and they can then choose whether to accept or reject the invitation.
In addition to receiving an email invitation, invitees can go to the I'm Invited view in the Communities Catalog and see the list of communities and subcommunities that they have been invited to join. Invitees can then click the link to Accept or Decline the invitation.