Me and my friend would like to create a game together. So is it possible to share a game with another user and both work on it? Even if not at the same time?

Right now it's not possible, no, unless the two authors swap copies manually. But I second the request. It would be very useful to more skilled users tutoring learners.

If using the on-line editor, if you both have access to the same account you can both edit the game. Probably at the same time.

If off-line, you could try using something like Github as a repository.

You could also use something like a shared google drive folder to both have access to the same files

Sharing an account is hardly a good option, though. What if I want to work with someone I don't know all that well (and therefore can't fully trust with all my data)? What if I want them to own the finished game?

google drive allows you to set shared permissions on individual folders to another person, doesn't involve sharing any accounts or information beyond an email address. It's the method I prefer for multi person projects.

Sharing an account is hardly a good option, though. What if I want to work with someone I don't know all that well (and therefore can't fully trust with all my data)? What if I want them to own the finished game?

OP did specify this was with a friend. I would set up a new account for the collaboration.

Your objection applies to any collaboration system; you should restrict the other person's access to the specific data for the collaboration, and you are obliged to trust the other person with that data (though version control systems like Github will mitigate that).

Your objection applies to any collaboration system; you should restrict the other person's access to the specific data for the collaboration, and you are obliged to trust the other person with that data

Which is why an explicit collaboration mechanism, that allows people to decide exactly how much to share (and by extension what to keep private), is better than improvised solutions. What if I wanted to work with someone who doesn't happen to be a close, trusted friend? It's the case all the time in... oh, I don't know... school settings. Sure, making a new account specifically for that purpose sort-of works. It's also clumsy and wasteful. What if I want to work on many such collaborative projects, with different people? Do I make a new shared account for each and every one of them?

Alright thank ye guys.