Only Email/Print/Calendar admins and above can do this.
NOTE: Only accounts with verified Mobile/SMS numbers will receive texts.
In the side menu, click Org / Team Tools > Accounts.
Click Customize Filters.
Click Has Valid SMS dropdown and select Yes.
Select the account(s) to text.
If you do not see the desired account(s), it means they do not have a verified number.
Click Communicate > Email/SMS/Push Custom Message.
Check the SMS box and click Create Message.
Enter the message.
When finished, click Send Message.