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 Tools > Accounts.
Click Customize Filters.
Click Has Valid SMS dropdown and select Yes.
Click Apply.
Select account(s) to text.
If you do not see the desired account(s), it means they do not have a verified number.
Click Communicate > Send SMS.
Enter the message.
Click Send.