HGG Community Forums

If you find them via the search user, you can open their public stable page by clicking on their name.
On top you'll see "find them on the forum: 'Name'". If you click on that name you'll go to their forum page.
On that page, on top in the left corner you'll see 'send 'name' a message'. If you click on that a new message will be created.

Another way to do it is to go to your Inbox here on the forum, click 'start a new conversation' on the right and then type in the name of the user. However, there are many user names and some are very similar so I always prefer the first method, as you will be sure you are messaging the right person with this first method.