There are three ways to add new email recipients:

• Using the Catch-all feature
• Manually adding email recipients
• Using LDAP sync

Or, if you want each user to have their own quarantine login, the following articles may be helpful:

How do I setup Quarantine Users?

Can I add quarantine users in bulk?

Using the Catch-all feature

When service is first activated on a domain, the catch-all feature is temporarily enabled, making it unnecessary to add email addresses manually.  Email addresses that are detected are added to the detected on the Settings > Email recipients page:

After the 30 day trial is over, the catch-all is automatically disabled, unless you implicitly enable or disable it sooner than that.  See: Should I turn off the catch-all feature?  You can optionally move detected email addresses to the Approved or Invalid tabs by selecting one or more email addresses and clicking on these buttons:

Email addresses do not need to be moved to the Approved tab to be considered valid.  Email addresses that are marked as invalid will be cause further email addresses to those addresses to be rejected (and will not be stored).

Manually adding email recipients

Email addresses can be manually added by going to the Settings > Email recipients page and clicking the Add recipients button:

You can also add email recipients in bulk by entering each email address, separated by commas in the entry box:

Using LDAP sync

See: Using Active Directory Exchange or LDAP to set up valid email addresses.