Soda posted a note about the experiencing sharing after carrying out some investigations on how close you need to be to other players to share experience gained.
Based on the findings, he put together an image which illustrates that you need to be within about 35% of a mini-map distance to share the XP.
To put this into perspective, Risingred also put together a small image to illustrate the distance a little better.
Bashiok’s reply uses ‘screen’ instead of ‘mini-map’ as the benchmark.
When playing with others it would seem very counter-productive to not stick together, there’s not really any reason to be playing with other players unless you are wanting to group-up and share the experience any way.