Several grids such as the Constituent, Events, Event Attendance, P2P Dashboard and Auction allow for you to send out Test Messages to any Constituents with a mobile phone number on file. Once on the grid you are able to filter for the desired Constituents and Select Actions>Send Text Message.  The top field will allow you to insert a mail merge field value(s) into the text message. You can also choose  whether you wish to send the Text Message right away or schedule it for a later Time and Date.