There are a few multiplayer mods available for Portal, but if you want to revisit Valve’s classic puzzle-platform game with friends, this one, with a pretty straightforward name, could be a great place to start.
Released back in 2023, Portal: Cooperative includes a two-player map set, a two-player Rexaura map set, an experimental three-player map set, and a playground map for testing ideas. Here’s a video from Jack5 to give you an idea of what it’s like to play with three people:
