The activity for a requirement can be found in the specific requirement's details window under the Activity tab. Activities will be listed with date, time, and user who performed the activity. Activities include changes made to the requirement which include version creation, enhancement request creation, associating upstream and downstream requirements, adding comments/reviewers/contributors/co-owners/attachments, linking tasks and bugs, etc.
Users will need project permissions to view the requirement. 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. Users added to a specific requirement as a contributor, reviewer or co-owner will be able to view it.