Write the message you want to be sent out to your clients, and then you can select which clients you want the message to be sent out to. Your client list is broken up so you can see which of your clients have incomplete sessions remaining with you and which clients have no sessions remaining with you but may want to rebook more.
Once you select your clients, send the message. When you return to your inbox, you'll see that your message has been sent to each of the clients you selected.
Note that this is not a group message, so your client's replies will only come back to you.