Sometimes if I join someone else's lobby it doesn't let me select certain slugcats. Recently this is happening with Rivulet.

EDIT: Figured out a solution, though I'm not sure why it works.

Apparently it has something to do with the last campaign you personally ran as a host. I had previously been running a Riv campaign as host so after I started joining others' lobbies afterward I was unable to pick Rivulet.

So the solution is to host a new game, set the campaign to a slugcat you never want to play, then you should be able to select any of the others.