Guest users are logged in to the Guest Portal and the free license allows them to participate with:
- Add new issues and requests and Edit them.
- View assigned tasks, issues, and requests but NOT edit if that activity was created by another user.
- Modify personal settings.
- View project details (provided you are assigned to an activity on the project, it is not closed, and has not the Draft or Template status).
- Upload files to assigned activities, regardless if activity was created by Guest user.
- Use activity message boards regardless if activity was created by Guest user.
- Mark their part of assigned work as done.
It's NOT possible to modify object permissions for a guest user.
Such users do not require licenses and the number of them is not limited.
Some important fields will not be editable to the guest such as status or progress, even if created by the guest, including being "team" members, on their user info/profile page.
Any assigned work will NOT include the Guest user in any workload reporting.
Further reading: Guest users