Almost all monsters can be shared with friends. Summon any multiplayer monster, complete at least 1 round, and then share it with your coven.
You can share a monster by publishing it to your timeline, inviting friend by sending them a social network request, posting an invitation to your coven, or sending a special link to your friend.
The "Share" button is at the bottom left corner of the battle screen and also at the "Round Finished" screen. Click it and choose the invitation type. If you are the summoner, you can also add a message for fighters.
The invitation types are:
Post to Coven
If you're a member of a coven you can invite members of your coven to join the battle by listing the monster at your coven page. To call your coven please click the "Post to Coven" button. Your monster will appear in the corresponding block on your coven page and other members of your coven will be able to join the fight. NOTE: You will see the monster in your "Active" tab of the "Monsters" page. The "Coven Monsters" page only contains monsters summoned by someone else that you haven't joined yet.
Invite Friends
Inviting friends using social network requests gives you more control - you can choose whom to invite to join the battle. To send requests please click the 'Invite Friends' button and select friends in the invitation dialog. Your friends will see the invitation in their social network notifications and their Inbox within the game.
Make It Public
You can also make a monster available for everyone. To do that click the "Make It Public" button. The monster will appear in the "Public Monsters" list and any player will be able to join the battle. Please note that monsters can be private (only the summoner can see them) or public (everyone can see them). By default all monsters are private. You can only invite other players to multiplayer monster battles. Only one player can fight solo monsters, and altar monsters have a fixed list of fighters.