You can add multiple people to multiple projects using the Mass Update feature.
- Click on Settings, and then Projects.
- Check off the project(s) you would like to add users to.
- Click on the arrow beside the Mass Update button at the top of the page.
- Select Add People.
- Select the people you would like to add to the project(s).
- Click Apply.
You have successfully added the selected users to multiple projects using the Mass Update function.