User access to a given study can be managed such that only users who are added within the Study > Configure > User Access module for that study will have access to the study.
Each user who is added here is restricted to a single Role which will drive their interaction on the study; and the defined role must also exist against their volunteer profile.
Once at least one user is added here, users who are not added here will not have access to the study. If no users are added here, then access to the study will be determined by the Study and Site restrictions associated with user profiles within the Administration > Users module:
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After users are imported into the User Access section, the following actions can be taken:
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More about how these user access restrictions relate to user access defined in User Profile is described here.