The easiest way to become another member's "Friend", is to click on their "User Name" and go to their "Profile Page". You can find their User Name if they're online, at the bottom of the forum homepage, under "Current Active Users"...or, you can go to one of their posts and click on the User Name there, then click on "View Profile".
Once you're on their profile page, just click on the "Friends" tab, and then click the "Befriend" link. Or, you can just click on "Add As Friend" on their Profile Page.
You'll be notified of an incoming friend request through "Notifications", which is found at the top right of the forum page, next to your User Name. These requests can be either accepted or rejected.
Your friends will show on your profile page, check their Avatar to show Avatar when accepting the request. Your Avatar also shows on their Profile Page. Your friends will have a + sign next to their names at the bottom of the forum page, under Current Active Users.