If you wish to join this league, just write your nick and click Join.
Why don't you login or register for Sim Racing Manager? If you do, not only will you be able to create your own Groups, Divisions and Championships, but will also be able to follow the Groups you are part of, points earned, among other features.
If you do not login or register, you can still join this Group. However, if at a later date you wish to link this entry to your account, the Owner of the Group will need to send you an email invite.