For casual co-op experiences, I'd recommend checking out Overcooked! It's a chaotic cooking game that requires teamwork but is easy to pick up. The levels are designed in a way that gradually introduces challenges, so even non-gamers can have fun without feeling overwhelmed.
Another solid choice is Castle Crashers; it combines simple beat-'em-up mechanics with a charming art style and humor. The gameplay is straightforward, making it accessible while still providing enough depth to keep everyone engaged as you progress through the game together.
I'd be curious to know what genres your friends lean toward. Would they prefer something more action-oriented, or are they open to different styles?