A Group alias (or Email alias) is used for Email to SMS.
When creating a group in SMS Central, you'll have seen the "Group Alias" field.
You can use the group alias to send the email to firstname.lastname@example.org. You can change it as you please, but remember that the group alias cannot contain spaces or special characters.
The group alias allows you to send the email to sms to email@example.com. When writing an email and sending it to this group, all the contacts contained in this group will receive an SMS.