Using your Sauce agency account you can 'Invite a brand' via your Agency dashboard. Once your client brand is invited you'll be able to manage their Sauce account for the purposes of:

Use your main menu > click to Invite a brand

Fill out the form to identify the brand you wish to invite:

You'll be notified via email once your invitation has been accepted.

To see a list of your invited accounts > from your main menu top left > click on See all invited accounts:

The list of invited accounts shows you the brands you have invited and the status of the invitation - see below

You can resend an invitation by including the Invite link (shortlink) provided on the right of the list:

Once a brand has accepted your invitation you'll be able to manage their account by clicking on their @username from the list of invited accounts:

Assuming you have enabled all the required features for your client you can now manage their account to:

Go to the top left menu > Click to your agency @usernamne