The organization selection page shows you all organizations that you’re a member of. It also shows you organizations where you’re a contributor or have read/write access to a project on the organization (even if you’re not a member).

This means it will also include tiles that display the username associated with a user that has added you as a collaborator on one of their repositories in GH. It will display in the UI as a tile with just the GH user's name and profile picture.

Additionally, you can read more about how CircleCI handles permissions in our docs: