In single-user Relay, there is a single user per account.
In multi-user Relay, the account is initially setup with a user the name and id of which are related to the organisation. This user is known as the account holder. Additional users are created for the account so that different people can sign in separately.
Users can be arranged into user groups, to make management of them easier.
Only the account holder can create and manage users. See Giving administrators user management permissions if you want other administrator users to be able to do this.