A lot of solutions do allow public registration (if this isn't an organizational project--most of mine are, for example), or for example, if you use a solution like Gitea or Gitlab, it supports mirroring a project on Github (if you want the public to contribute, this can be a helpful pipeline in) and you can aggregate issues and pull requests that way.
Gitlab, in particular, does support Single Sign On (via Google and Github, I believe!)
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A lot of solutions do allow public registration (if this isn't an organizational project--most of mine are, for example), or for example, if you use a solution like Gitea or Gitlab, it supports mirroring a project on Github (if you want the public to contribute, this can be a helpful pipeline in) and you can aggregate issues and pull requests that way.
Gitlab, in particular, does support Single Sign On (via Google and Github, I believe!)