People can submit a request to join your team.
Accept or deny pending requests
To accept or deny pending requests, go to your team in the teams list and select More options 
> Manage team > Pending Requests.
Request to add a member
To request someone be added to a team you're already a member of, go to the team in your team list then select More options > Add member. Type the name of one or more people then select Send request. Team owners will receive an alert that they have a pending request.