^ You're Welcome.
When you create a private message the recipients name will autofill, giving you a list of names to choose from. After selecting the username that you wish to send the message to by clicking on that name, it will automatically have a semicolon ( ; ) following the name as well as a space after it. From there you just type the next name and it will autofill again, you click on the correct username to add, and repeat until you have all the recipients that you wish to have for that private message. It will still work fine if you have a semicolon after the last name on your list, which will automatically appear when using this method.
You can also do this manually, but I prefer to use the method that I outlined because it will ensure that you have the correct spelling of the username, as well as the correct format for multiple recipients. Using the moderators of the Support forum as an example, the recipient list would look exactly like this:
Enix150; animal_cookie; Tommyboy;