Any user on your Seller or Vendor Central account that has, at minimum, user permissions to 'view and edit' Campaign Manager, will have the ability to log in to Flywheel.
Only Account Owners of the Seller/Vendor Central account will be able to invite new users to their account.
Here are steps for adding a new user:
Login to Seller Central as the account owner, toggle to the desired account, and navigate to Settings > User Permissions
Enter the contact information for the new user and click Send invitation. Repeat for all new users you want to add.
The invited 'user' will receive an invitation email from Amazon. Please advise them to follow the instructions to create a new account. They can log in with an existing email or an email that is already affiliated with another Amazon account.
Once the 'user' has accepted the invitation, log back into your account to grant the 'user' the correct permissions. User Permissions > Edit (next to the email of the new user).
Within User Permission List: Advertising section > select 'View & Edit' for Campaign Manager. It should look similar to the image below.
Once this is complete, new users will be able to access your Flywheel account.