You may have noticed that you have a section named Friends and Foes in your profile.
Friends
This allows you to make 'friends' with other members on the site, and allows you quick access to there profile, posts and to send PM.
Foes
This allows you to essentially ignore another member, from my understanding there posts or the majority of them do not show when you browse and they can not send you a PM.
To request a friend - or add a Foe
Go to the profile of the person you wish to add, by clicking on there username in either a post, in the members that have been active or by using the memberlist function.
Just above the profile picture/avatar you will see two sentences, add to friends list and add to foes list, click the appropriate sentence and then the request will go to the member.
To accept you must go to profile, then look at the tabs across top (which turn purple when you scroll over) go to Friends and Foes, scroll down to see the different sections, friends list, foes list, requests and requested and click the green + to accept or the red - to decline.
Give it a go - check and see if you have any outsanding requests and make a few requests yourself - have fun!!