How to work on a project with more than one person

We want to work on our project but if one of us is working on a scene in this project, the other cannot open the project to work on another scene. Unity gives an error at start. “No no, bad girl, there is someone working on this! You may not come in!”
How can we do? Or: What are we doing wrong?

I would pretty much want to know the answer to this too.