While creating your guests, there are different categories. Some guests are service providers some are part of the retinue. Also they may be attending or not attending depending on the reply to the invitation. Based on the categories of guests,

you can send specific emails to them.  Click on SMS & Email and then New SMS. The following will be displayed:

You can also include additional recipients to the SMS. This is useful when new guests have been added to the website after you have sent SMSses to the other guests.

This additional recipients text field is auto-complete meaning you do not have to type the full name of the guest. Start typing the name of the guest that you wish to add and select the name from the drop down menu:

Then enter the message, and click on Send