Community managers can invite you to join a community as a member. It is not yet possible for you to request to join a community.
Go to My requests where you will find the invitation (click My dashboard in the header, and afterwards My requests in the tab-menu).
Click Accept or Decline directly on the invitation, and confirm in the dialog you want to accept/decline and optionally provide a message to the community managers.
Click the invitation request to see details about the request and have a conversation with the community managers.