In the specific requirement details window, click on the Add Contributors button to view the list of available project members to add. In the pop-up, click on the project member's name to select them and then click Add to finalize. If a user is not appearing in the list of available users to add, ensure that the user and their organization are added to the project first.
If adding the same contributors to the multiple requirements, the Group Actions feature may be used.
Requirement owners and requirement contributors of that version may associate contributors. A user must have view and edit permissions in requirements to associate contributors. Depending on the settings of the user's project permissions, they may be able to view all the project requirements or just a subset of them. The same is true for viewing the list of project members when adding project contributors and reviewers. Users added to a specific requirement as a contributor or reviewer will be able to view it.