To add a Dashboard Viewer to your team, simply go to the team management page in your profile, type in the user's email address, and choose the 'Dashboard Viewer - free' license in the dropdown. Once you have added this new member, an email is sent to the user to activate their account, and the individual can then view the dashboards you choose to share with them.
To remove a Dashboard Viewer from your team, simply go to the team management page in your profile and click on the dropdown next to their status. This will give you the option to remove the user. As soon as you remove this user they will immediately lose access to the dashboards you had previously shared with them.