You can let other people have an address at your domain in 3 ways:
Adding another delivery group lets you forward mail sent to a particular address at your domain to another person. That address is part of your account, which means they will use your spam filtering, and any other settings you have set up. The other person doesn't have a separate account, and they can't log in (unless you give them your password.) It's best for someone who you think will only get a small amount of mail at your domain. It requires a Pobox Plus or Mailstore account.
To add additional accounts that you pay for to your domain, you can add a new account to your billing group.
This will create new accounts that are added to your bill. You can add or remove new accounts at any time, and assign them to anyone you wish in your organization. These accounts have their own passwords and settings, and are completely independent from your account.
If you'd like to let someone else use your domain, but you don't want to pay for their account, you can send them an invitation. You can specify whether they can choose their own address at your domain, or if they are limited to a particular address.