You can invite a user with a user role in Orangescrum. During the invite, you can add the user to selected project(s) as well.

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Note: The invited user will get an email notification and once the user signed up, s/he will be assigned to the user role you selected. As per the user role permissions, the user can perform the actions in the project.
Tip: While inviting, you can’t assign multiple roles to a user for different projects. However, you can assign a different role for a specific project.