The short version is that Subnautica 2 is built around optional online co-op as well as solo play. If your real question is whether you and your friends should plan around multiplayer, the answer is yes. If your question is whether your specific session is working, use the fix page next.
If you are buying the game mainly to play with friends, the design direction supports that. The main caveat is that Early Access co-op can still behave like an Early Access feature: updates can disrupt sessions, platform builds can drift, and one player being out of sync can break the whole plan.
If your only question was "does it have co-op or crossplay," you already have the answer you need. If your actual issue is that invites fail, players cannot join, or one friend keeps getting left out, jump immediately to the multiplayer fix guide.
