There is no Private Messaging feature on the site at the moment, but you can get in touch with other members via their "personal contact form", if they have that setting turned on.
Every registered member has a personal contact form on their profile.
To turn your contact form on or off:
To find another user's profile page, you can either click on their username in one of their posts, or search for them.
To search for a user:
If that user has turned on their contact form, you will see a "Contact" tab on their profile.
Unlike the Private Messaging system of the forums, personal contact form messages are sent "via ARK" from your email address to the recipient's email address. That is, your email address (as the sender) will show up with your message in the recipient's email inbox. However, you (the sender) will not see the email address of the recipient until the recipient replies to you.
When receiving a contact form message, Google (gmail) adds a warning that "this message may not really be from the sender"-- that's because ARK is acting like the "middleman" between two email addresses. It's good that Gmail points this out, but it may cause some messages to get filtered into spam boxes.
Finally, my own personal contact form is available on my profile page and also through a link in the footer. Right now, you don't need to be a registered user to contact me (but you do in order to contact other members of the site).
If anyone is having trouble sending or receiving messages through the personal contact forms, please let me know by posting a comment here. Thank you!