It is possible (I've been able to do it before), but it is VERY difficult depending on your desired throughput and number of XBees trying to talk to each other.
In order to get it to work, you need to plan out very carefully what each module is going to say and when they will say it. Then you can plan out a timing sequence so that each XBee has more than enough time to listen or talk without clashing with the other XBee. In short: the timing of each transmission is extremely important.
The fewer the XBees and the smaller the throughput, the more success you will probably have.