There is no limit to the number of users that a store can have. To add a user you need to go to Settings> Manage Users > Add User.
Users are able to have different access within the system, below is a list of roles & its permissions:
There are some email prefixes that are blocked by Seller Center, if any of the below are used an alternative will need to be added.
If you have multiple Seller Stores, your email can be set up as a switchable user so you have the ability to switch between stores without having to log out and back in under separate email addresses. This can only be done if there are no Oauth Application/IDs linked through your user your account.
Please check the link here to see if you have any Oauth details created and if there are none, please submit a ticket via the Help Desk with your email address and the store/s you require access to and this can be amended.