Why there is no solo mode
The game is designed around teamwork and talking. Cooperative exploration, puzzles, proximity voice chat, gestures, and communication tools are part of the core experience.
Can you enter alone?
The official FAQ notes that a person could start a game and walk around a little, but would not get far. That is not a supported single-player mode.
Smallest supported group
Two players is the official minimum. When hosting, choose the 2-player version of the world for the smallest supported group.