Guests will only be able to view and edit the design that you share with them. They cannot access your brand account.
Here's how you can add or delete guests in your account:
From your dashboard, click the arrow on the bottom of your left side panel, and select Team Members.
Enter the email of your guest, then on the second row select Guest. On the third row, select the design that you'll permit for them to access.
And lastly, click the Invite button, and wait for them to access the sent email.