Group actions can be used to add tags to multiple requirements at the same time. Note that tags will need to be added to the project first, which can be done in the Project Settings tab by a project owner.
Click on Group Action, and then select Add Tags. Select the requirements from the potential list, and then click Next. Select the tags to add, and then click Submit. Those tags will now be added to the selected requirements.
This can also be down in the requirement tree view to narrow down the requirements by parent entity. Right click on the parent entity and select Group Actions, then select Add Tags from the drop down.