As a member of the "Admin" user group, you have the ability to add new users and groups to access the application.
Adding Users is straight-forward: you supply basic information about each user, such as name, email address, and groups they have access to.
Visit Settings > Users:
Adding Groups is where you apply fine-tuned control over various aspects of the application. For each group, you can designate which lists are accessible, the default sending method, and many application-specific permissions and upper-limits.
Visit Settings > User Groups:
Also, under Settings > Mail Settings, you can create individual connections for each group, if need be:
Groups help you maintain a subset of users' rights and privileges. You can organize your users according to their job function, and use their group settings to give them access to only the necessary parts of the application.
If your users are separate clients entirely, you could consider groups as "accounts," where each client is assigned to a group with specific control over aspects of the application. Perhaps each client is designated separate SMTP settings, etc.