Yeah it got broken, probably bit. This happens easily during typical group activity. The young ones will explore and test each other and a tail bite can also be a more purposeful/offensive gesture as well.
As long as it is still getting blood flow it will be ok. If the hair falls off and it looks desiccated, then it may need amputation at that joint. http://www.sugarglider.com/gliderpedia/index.asp?Amputation