Team members allow you to add additional accounts that have the ability to send notifications on your behalf. Once you add a team members, they will instantly receive an email with login details.
By default, team members do not have access to view all of your sites. In order to share website access, head to "Websites" -> "View Websites" and click edit. Once on the following page, click the "User Management" tab and add the email address of the newly created team member.
If a team member updates their account to a paid account, they have the ability to create their own team and as such will be automatically removed from yours. They will still be able to access any websites that you have shared with them.