Multi-party key agreement
Historically, both the difficulty and the risks involved in securely establishing shared keys in large networks has led to the invention of public-key cryptography. So you might first want to consider using digital signatures/asymmetric encryption instead of sharing the same secret between parties. 

If you need all (or some) of the parties to collectively sign or encrypt a message, consider using threshold signature schemes with a Distributed Key Generation protocol. That will produce a key pair that is distributed among $N$ participants such that at least $k$ of them need to collaborate for performing a key operation. For discrete-log cryptosystems there's a paper by Gennaro et al that describes such a setup.

Edit: If you only need to establish a common secret (instead of a key pair) between all the parties, a simpler solution would be to use a generalized Diffie-Hellman key exchange instead. This works for honest participants with insecure communication channels. If the participants can be malicious, a more robust solution is described by Tseng, 2005.