Hi - We have something called the Collaboration Package. This gives us Reviewer licenses. A Reviewer license doesn't cost anything, as it is part of the Collaboration Package. We use Reviewer licenses for, among other things, assigning them to third parties who have need to create and contribute to issues. We can assign them to tasks. The only thing they can't do is charge time to timesheets, alter tasks, and so on. You need a Plan or Work license for some of that.
This site explains the license types:
https://support.workfront.com/hc/en-us/articles/216669668-Understanding-License-Types
(from the help file)
Review
The Review License is part of the Collaboration Package. The purpose of the Review license is to provide permissions to resources who are not project owners/team members but need to access Workfront to see all of the items they are involved with. For example: A stakeholder participates in an ongoing legal review of marketing materials as part of the work process. The Review license allows these resources the ability to log in to Workfront to see updates on work. In addition, they can participate in the work process by approving projects, tasks, or issues. They are not able to approve timesheets.
The Review license is essentially a read-only license type, with the ability to approve. A secondary use for this license type could be to provide limited access to external project stakeholders (i.e., vendors, subcontractors, clients, etc.) that may need to see the timeline, status of a project, or approve information but do not need to have permission to collaborate through the system.
If a user with a review license is assigned work, the manager will be prompted with a warning. The warning is a reminder that the user will not be able to edit or complete the task or issue. The warning will also show on tool tips to aid in the assignment process.
Hope this helps!