In the older versions, a simple way to add more then one person from CRM to an E-mail was to use the E-mail window pop-out option in your Preferences and, click the E-mail address from a Person record which submits into the To field of the popped E-mail window.
You could then position your cursor into the E-mail window CC field and browse to another persons record in CRM, click on their E-mail address and it would also be submitted to the open E-mail window into the field where your cursor is sitting ie. CC field.
Now, in both the 2017 and 2018 versions, it overwrites the original E-mail address submitted in the To field and does not allow you to add another E-mail address to the same E-mail window. Unless, there is a new method of adding random E-mail addresses, which I haven't found yet, is this correct behaviour?
Regards,
Penny