You can add current users or invite new users to your group at any time through the Bitly Brand Manager. To add a user to a group or invite a new one you must be a Group Admin or Account Admin.
Navigate to Account Details section by selecting the Organization Settings located in the Menu.
In the Organization Settings navigate to the the "Users" section. And then select "Invite New User"
From there you'll be prompted to the invite user page. Enter the email address or username connected to the account you're inviting. Also select the groups you'd like to give them permission to by checking off the desired boxes.
Select "Invite" and the user will be added to your groups.