Select guitar/drum first, then search song by name
On official site's フレンド編集 page, select guitar or drum first, then use Gitadora ID search the player who has the song you want, and add him as a friend
Then you can play these unlocked songs at game center! However, you can only get 3 songs randomly (every play) from another player unless you two are mutual friends, so we recommend you add multiple friends for one song if you really really want that song.
How to share your song
Edit on official site's 『フレンドにオススメ』編集 page, and upload skill as usal, no extra steps! (About how to upload skill, please refer to 'How to use' section on home page.
According to Konami official, you can only get 3 songs randomly (every play) from another player unless you two are mutual friends. Notice that you can only share songs for GuitarFreaks and Drummania respectively.