To join a circle you first need be invited by a circle lead or club administrator. When you are invited to a circle, an email notification is sent to your email address with a link to either accept or decline the invitation.

You can also view any pending circle invitations by going to your Profile page and clicking on the Circles card to open it. From here you can accept or decline circle invitations directly:

Once you accept an invitation you will be a part of the circle and from this screen you will also be able to manage your circle permissions.

If you decline an invitation but then change your mind, you will have to be invited again by one of the circle leads.