Local network in a (mobile) signal less area - unsolved problem?

Say you are a group of 50 people going for trekking. 

Now, you are in a mountain/ forest area where there is no signal. 

Will you be able to communicate with each other using some sort of local network ?

..maybe wireless / bluetooth / radio frequency?

Given that mobile phones have too many parts(wireless component, bluetooth component, radio component??(not sure about this) will you be able to communicate only with each other (using local network) ? 

There may be already some ideas based on this.. posting it here for the world to think about.
BTW world includes me ;-)

--

Note, it should not be just 1:1 communication by bluetooth/ it should be like everyone should be able to communicate with everyone else..(ok.. think there is some cap like 20 almost..but people should be able to join/leave the network)

--