From what I've been able to pick up, there's a brief bit of dialogue at the end of the maps with the characters finding the talisman, then it talks to MU for a bit.
And yes, they join the main party as any other unit. The only difference is, they're counted as bonus characters and build up towards your limit of 20; getting more beyond that requires dismissing one. You can, however, save them to the Unit List to preserve them and share them between files, but for all files, there are only 99 slots available.