What is the difference between a sub-account and a user?
A sub-account can contain contacts, templates, and settings. It is the sending entity that can be accessed by one or more users (see the diagram below for a basic visual representation of the relationship)
A user is a profile that can be invited to one or more accounts. You should no longer have to share login credentials between users in your team as each user should have their own credentials. If a team member leaves, simply remove them from the account and invite another user if required.