Only the account Owner or Admins can edit Guest permissions.
You can control which workspaces your guests can access by following these steps:
Click on your workspace name in the top left of your Motion workspace
Select Workspace Settings
Under Organization, click on your organization name
Under Users, click the View button
Select the guest user you want to edit
Click on the Workspaces tab
Toggle workspaces on/off the workspace(s) you would like to grant/remove access to
Your changes will be applied immediately, and the guest will only be able to access the workspaces you've enabled for them.