Hi EleniBraley,
I'm Charles, an Independent advisor trying to help.
When adding a user, usually to the right or left depends on the application is the + button to continue adding? You do not appear?
Go to Messaging >> Click on sign (+) >> Search / write the name of the first contact, then >> Search / write the name of the second, by clicking >> Sign (+) again >> Define the content> Send the SMS.
All related information is in this article, which does not have a section for sending SMS to multiple people
https://support.microsoft.com/en-nz/help/445935...
I hope these steps also work for you
Attentive to any comments
Have a happy day
Best Regards,
Charles.