Instructions
Log in to your Roblox account. If you don't have a Roblox account yet, you can create an account for free at roblox.com/Login/NewAge.aspx.
Click the "People" link at the top of the page. It is the fourth link from the left, between "Catalog" and "Builder's Club."
Enter a username, or part of a username, into the search box. Click the "Search Users" button.
Click on a username in the search results list. This will take you to the user's profile.
Click "Send Friend Request." You can find the link near the bottom of their profile, next to the "Send Message" link.
Enter a message in the text field. Click "Send." This will send a friend request to the user. If they approve the friend request, they will be added to your Friends list.