You need to go to their profile page, such as:
Then click +Connect in the top right
If the user is a member of element14 Community staff, such as cstanton the Community Manager, you should be able to send them a private message. However if they're another user, you'll typically have to 'Request Friendship' first.
You should be notified when they accept. However, you won't be notified if they reject your invitation.
And the best method I then use is, go to the speech bubble:
Click on 'New Message'
Type in the username(s) of those you want to send the message to:
Then you can type in your message, and click 'send'.
You should always be able to send a message in this way to element14 staff and administrators.