So you're browsing the forums and someone says, "If you're interested in this game, drop me a PM!" and you may be wondering, "Well, how the heck do I do that? And WHAT is that?"
You can send a Private Message (or PM) to any other user on Roll20.
The quick way to send a PM is click on a username in the forums or Player Directory. This will take you to the person's profile page. Now click on 'Send Private message'. This will bring up an interface much like the one for writing a forum post. Click 'Send' to send the message.
If while writing a message you change your mind, click cancel. If you want to pause writing click on the X in the upper right corner of the message box; you can resume writing that message at any time by accessing your private messages via the 'Welcome, YOURNAME' link and clicking on 'New Private Message'
Group Private Messaging
You can send a PM to multiple people at once. To do this, from the Private Messages Inbox, and click on 'New Private Message.'
You will be asked to type in the name of the recipient and it will try to find a match as you type. Once you find a match, click on the name in the dropdown list it shows you. Then do this again for each recipient you wish to include in the private message.
If someone responds to a message you send, you'll receive an alert.
If your message inbox is getting cluttered, you can 'Archive' messages you're not interested in keeping around. This is effectively a Delete button; you won't be able to retrieve the message again unless another person in the message thread responds to that thread.
If you are receiving messages that break the Code of Conduct, or are simply from a person you do not wish to engage, there are features available.