The group alias is an optional field allowing you to specify an alias associated with the group for sending to the group from your email client.
See Add a Contact Group
Your email must be registered for Email to SMS (Registering for Email to SMS )
If you have a group with contacts and an alias AND your email address is registered for Email to SMS you can send an email to [alias]@mtxt.co.nz
The Contact Group will receive the contents of the email