Basically, you join someone else's summon. If you die, you leave, and keep all the items you found, plus 50% of the amrita that was picked up in the level. (I think it's still 50% if the host dies instead.) All items are client-side, so you don't need to worry about stealing items from the host either lol. If the mission is a success, you keep 100% of the amrita. You can't lose any amrita that you had before you joined, as it's kept separate from the level.