Depending on your Synthesia plan, you have different limits for the number of Guests you can have in your Workspace:
Freemium: No Guest.
Starter: 3 Guests.
Creator: 5 Guests.
Enterprise: Unlimited.
See the Synthesia pricing page to see all features included in your Synthesia plan and the Synthesia User roles & Permissions documentation to learn more about the Synthesia Guest role.