Druva has introduced a couple of enhancements in inSync Share in the 5.8 release. The enhancements perfectly address your requirement and should fulfill the requirement of your users. With this enhancement, users get to send custom messages or instructions when they share links. The email addresses are also auto-populated based on keywords if the recipients are also inSync users. However, you will need to upgrade your client to 5.8 so that your users can take the benefit of this enhancement.
You can see the 5.8 Release Notes for more information on the following page:
Also here is the inSync Share End User Guide:
This should definitely help you.