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There are 2 main ways to contribute actively with code to a repository one of this is to be a contributor and the second one is to fork a repository.
The way you take is to add a contributor to your repostiory so your friend can simply do a local clone (in his pc) and make some changes. The best way to follow this choice without making to many conflicts is to create branches for every user and feature that you want to add in order to create pull request after that branch will be pushed to remote.
If you need a help you can try follow the official workflow guide of github -> here.
For video guide you can use the youtube videos -> here.
For a workflow more complex and advance -> here.
I hope I have solved your doubts
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