Why can't I see the Fellowship Adventure after joining a new Fellowship?
To be able to participate in a Fellowship Adventure, you need to have been part of the Fellowship before the Adventure started. If you join a Fellowship after the start of a Fellowship Adventure, you will be unable to participate for that round, and you won't get any of the rewards.
Should you leave your Fellowship during an Adventure, any progress you made will be lost for you, and you won't be able to get any of the rewards achieved by the Fellowship.
Of course, you will be able to participate in the next Fellowship Adventure if you are a member of a Fellowship at the beginning of the event!