If you know the person's forum username (which is what I put in the welcome post) and want to sent a message, click the 3 bar icon at top right of the screen. Click the envelope, click compose new, enter the user name. As you type, or if the name is valid, you'll see a list of users appear and you can click the one you want.
Add a subject, type you message and send it. The forum software will send the message to their inbox and send them an email notification that they have been contacted. No personal data is shared so it's compliant.
If they don't want to be contacted, they can just ignore the message.