Along with a UI overhaul to the level select screen, Painter's Pets latest update allows for players to control multiple slimes from the same device! This should allow both people hosting games and joining games to add a new players using additional controllers of keyboard inputs from their devices.

Once in a game room you can either use the Envelope button to invite a friend via steam, or the controller button to add a new player that will be controlled locally, at which point you will be able to select a profile and team as normal.