Okay so I recently started to expand my island and i see an island right next to mine. I was unaware that it was there. I asked mods to come look at it and 3 of them did. But I never got a clear answer as to whether I can continue building on my island. My ign is lennick4. Here is a video link: http://youtu.be/BtM-tIKKXVo
You can only build out 49 blocks from your bedrock. Any more and it is considered bridging. Also please make the video either Public or Unlisted. It's currently Private so no one but you can view it.
You can build some more than 49 blocks, but, bare in mind, if the person restarts, the part of your island that is more than 49 blocks away from your bedrock will be deleted. But do not get too close to them. I'd say you could expand like 53 blocks in total or so, so you're not too close, and the other person has room.