We would recommend that you introduce your clients to Collaborate before you begin using this functionality.
Our suggested best practice is for you to use the next meeting with your clients as an opportunity to take them through the process of receiving shared documents and to make sure they add the no-reply@sharepointonline.com email to the safe sender list in their email.
We have drafted a sample email that outlines the secure process for clients and potential issues they may encounter. You can download the Sample Email for Practices - Collaborate.docx file (including the sample email) at the bottom of this help article to use as a draft for your client communication.
This information can be set up as an Email Template. Or you can also set this up, for example, as a PDF and include this as an attachment in the email you send before or when you share a document. You can also refer your clients to our help article for any initial issues they may have - Troubleshooting initial issues with Shared Documents or Shared Folders.