How can I add a team member to several projects at once?

You can simultaneously add a team member to different projects if this user belongs to multiple teams. So, let's see how to do it:

1. Go to Workspace and click Members.
2. Click a required member's name to open the Edit Member page. 
3. In the Team Membership field, select the teams you want to add a user to. 
4. Click Save. 
5. Go to the Projects tab.
6. Click the project name you want to add a team member to. 
7. In the Team section, click Add members to project team. 
8. Check a required team and click Add. 
9. Repeat steps 5 - 8 to add a team member to another project.