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.

(information from FAQ)
What is the difference between a friend and a contact?

A friendship is a mutual, two-way relationship between two users. It is initiated by one user and accepted by the other.
A contact is a one-way friendship. This could be where the other person does not accept the friendship, a friendship request is pending, or you do not wish to be friends at this time but want to show them as a contact only.

Where is list of friends and contacts displayed?

Your friends and contacts are displayed on the 'Contacts and Friends' page in your User CP. Your friends are also displayed in your profile page.

How can I request a friend, accept a friend or break a friendship?

There are two ways to request a friendship with someone.
The easiest way is to view the profile page of the person you want to become friends with and click the 'Befriend' link.

The other way is to add the person to your contacts first. Then from your 'Contacts and Friends' page ask to be their friend by checking the box next to their name labelled 'Friend'. Next, save the page.

If you know the username of the person you want to be friends with you can send a friend request using the box provided and clicking 'Add Friend'.

To accept a friend, go to your 'Contacts and Friends' page. At the bottom of the page you will see a list of pending requests. Check the box for the requests you wish to accept then check the 'Accept' button and save the changes. If you do not wish to be friends then select 'Reject' instead.

Breaking a friendship can be done in three ways.

From your 'Contacts and Friends' page, uncheck the box for friend for the particular person and save the changes. If you no longer wish them to be a contact either, uncheck both boxes for that person.
In your 'Profile Page', click 'Friends' and then click 'Break Friendship' next to the user's name.
In the other person's profile, click 'Friends' then find your username and click 'Break Friendship'.