As an owner of a community, one can send invitation to users to join the community. An invited user can see the invitation when the user logs into portal. The user then has an option to either accept or decline the invitation.
VISITOR--> invited--> INVITED/VISITOR--> accepts--> MEMBER VISITOR--> invited--> INVITED/VISITOR--> declines--> VISITOR
When the system is set up properly, an invitation message is sent to the invited users through email. In order to receive an invitation through email, the user must have email address properly configured in their portal.