Joining a Custom League
ESPN Custom and League Manager (LM) Fantasy Baseball leagues are non-standard scoring leagues which use modified rule settings. Lm Leagues are managed their league creator (League Manager). All LM 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.

ESPN Custom Leagues are leagues that use custom rules and settings determined by their league creator. Once created, however, the rules cannot be changed and the league is managed the same as any Standard league - using the custom settings. ESPN Custom leagues may be Public or Private.

ACCEPTING AN INVITATION TO A PRIVATE LEAGUE
Private leagues can only be joined if invited to play by the league creator. 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 creator can invite people to play in a private league.