The ability to open or edit a Workshop module is derived from a user's permissions on the Workshop module. This means that a user must satisfy the Organization and Marking requirements and also have a role on the module (directly, via a group, or a default role). By default, users need the Viewer role to open a Workshop module, and the Editor role to edit it.
Note that the ability to open or edit a Workshop module is separate from the ability to access the data, actions, or functions which may be needed to fully use a Workshop module.
You can use the Check access panel in the sidebar to easily check a user's access on a Workshop module. This will show if they meet the access requirement on the Workshop module, as well as additional data requirements to see object types, link types, action types, and functions.