Lots of 'new age' books and websites talk about 'raising your vibration' and from what I can understand, it is exactly as Rainbow Angel explained...the more positive you are, and the more good things you do, the higher your vibrations.

As for being grouped with similar vibrations, I think it just comes down to good vs bad. it would be similar to Heaven vs Hell...different vibrations. Your Guides would be of a higher vibration as would Angels.