If you only want to message one student-athlete, you can slide right on their name and click "Message" from the menu that opens up. This will open up any previous conversations between you and that student-athlete and allow you to send a new message.
If you are looking to send a message to a few student-athletes that are not already in a group, you can hold down (long press) on the first name you want to be included in the message and it will pull up a check box next to each SA (automatically checking the first SA). You can then check the box next to each additional student-athlete you want included in the announcement, then click the action menu in the top right, and hit "Send Announcement" from that menu.
The final way to send out an Announcement is from roster groups you have already set up. To do this, click on the "Groups" button at the button of the app, slide right on the group that you would like to send a message to, and then click the "Message" button that will appear on the menu.