Rules: Joining a Custom league
"Custom" FFL09 leagues are non-standard scoring leagues which use modified rule settings and are managed by their league creator (League Manager). All Custom leagues are private participation leagues. This means you will only be able to join if the League Manager emails you with an invitation to join.
ACCEPTING AN INVITATION TO A PRIVATE CUSTOM LEAGUE
Custom leagues are private and can only be joined if invited to play by the League Manager. All invitations are sent in the form of an email which contains a unique URL. This unique URL is the ticket to play in the league. To accept the invitation, the URL must be clicked on or it can be copied and pasted into your browser's 'Address' field. Upon a successful join, your ESPN.com MemberId will be associated with a TeamId and you will become that team's owner.
NOTE: Only the League Manager can invite people to play in a Custom league.