Go to your User CP, and in the left navigation pane, select "Contacts and Friends." Scroll down to the bottom of the "Contacts and Friends," and you'll see any pending Friendship Request Invitations. Check the boxes next to the names of any requests you wish to accept, indicate that you wish to "Accept Request," and press the proximate "Save Changes" button. Once you do that, the individual's name should appear in your Contacts and Friends list, with the notation that the person is a "Friend."