Repeated Login Requests when working on repo

[D. Roberts]

Problem: 

I have a repo that I want to share with a team member. When he accesses the repo and tries to do any kind of basic git action on the repo it asks him to enter his password. But when I am working on the repo, it doesn't ask me to login. We cloned from the same URL and are trying to utilize SourceTree. 

Do we have to use SSH? If we do, is this going to impact their usage of GitHub if they were using that in the past? 

Thanks,