Right now, you can check out what your Xbox Live friends are up to from your phone, but you can't actually manage your friends list from any of the mobile Live apps (and Xbox Live Anywhere has gone MIA again). But the developer of one of the better ones, 1337pwn, says that Microsoft has made a change to the site that makes it possible to add friends remotely, and he thinks “that we can get it working,” though he's not making any promises yet.

Source:→ Gizmodo