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Open your local repository in SourceTree. Your local folder for the project shouldn't have the README.md yet. Click on the Pull button
Now, if you check your the folder of your cloned repository, the README.md file will be in it. Pulling is how you can get changes from other people from the remote to your machine. Make sure to pull before pushing.
Tips For Working with others
Have a single main scene and separate work across different scene
Serialized data like a Unity scenes or blender models are very tricky to work with when multiple people are making changes.
Find a way to separate concerns of work, such as make a scene for each person in the project and they build the systems needed for the project and push their changes to that scene alone
Then someone can pull those new Unity objects and blender models to the main scene.
Avoid working on the same prefab, script, or Unity object
Merging can be very tricky, it’s best to not have to do it when it comes to game development projects
Discuss who’s going to work on what and when teamwork is needed, use a video call and screen share if multiple people need to touch the same script or game object
It can be very easy to undo someone’s work on a prefab when multiple people are interacting with the same prefab and adding new components to it.
Additional resources
This is a great guide for setting up Unity and Git with a GUI:
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