A quick guide on using the Sharing Gamertags application.
Click on the Sharing Gamertags link in the navigation menu.
By using the Search feature (top right side of user list) you can specify the type and style of player you wish to seek.
Proper etiquette dictates that you should message them on Hupit via a Personal Message. You can do so directly while you are viewing their profile.
If you do not get a response, perhaps send them a message via your Console or Multiplayer site rather than sending a friend request.