Multiplayer is unfortunately unlikely to be able included in the game, at least in its initial state. If it's possible to make it work it'll be added at some later point.
Of your suggestions, creature sharing is a definite anyway (you can even share saved games, planets or whatever to challenge your friends for survival), and the only other one I could see working is symbiosis. Players would be limited to a single stage, and even then issues with entering the editor at different times would make it difficult to run.
Still, it isn't ruled out completely. We just need to find a viable way of making it work.