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.
Click Apply.
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.