ESPN Custom Leagues may be Public or Private, determined by the League Creator when creating the league. All League Manager leagues are private participation leagues. This means you will only be able to join a LM league if the League Manager emails you with an invitation to join.
ACCEPTING AN INVITATION TO A PRIVATE CUSTOM LEAGUE
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 Member Id will be associated with a Team Id and you will become that team's owner.
NOTE: Only the League Manager can invite people to play in a LM League.