You need to implement a communications protocol, which might be more or less "simple" or extremely complex. If every node is reachable by other nodes so that all the transmissions are possible by a single-hop, then it is certainly simpler than allowing multi-hop transmissions.
One way of doing such a system would be the following: all the nodes will work by default on a single channel (let's call this channel "control channel"). One of the nodes will be a network coordinator. It will periodically transmit a "beacon", i.e. a broadcast packet that will clock the network, and it might also include messages addressed to one or more of the nodes (every node, including coordinator, will of course have an address).
From the beacon on, the time will be divided in slots, and each node is assigned a slot number in which they can transmit. (Of course it is simple to manually assign to each node a time-slot than assign it automatically by the protocol). At every slot number the given node has an opportunity to sent a small message to any of the other nodes, or to the coordinator.
If it needs to perform a long transmission, it would sent a channel allocation request addressed to the coordinator. The coordinator will then, if resources are available, communicate the channel number to both parties. Then the two nodes will exchange information using this channel. Of course they will need to switch to "comtrol channel" to receive the periodic beacon (when it is close to happen), and then can come back to their "data channel" until the long data transmission ends and they can free this channel (also requesting it to the coordinator).
This way you can have several simultaneous data communications, as long as enough free channels in this band exist.
In the other hand, the protocol will need to be fault-tolerant to some degree. Probably your protocol will need to implement "acknowledgment messages", "message retransmissions", "channel allocation time out", etc.
Finally you might take a look at Zigbee protocol, or to propietary "mesh network" protocols as Telit NE-50 (which is described in its user guide http://www.telit.com/index.php?eID=tx_nawsecuredl&u=0&g=0&t=1427099902&hash=13d9f98e445e87eb5dda66cdd3a1a90c3e280fa5&file=downloadZone/3962.pdf) to get more ideas of what can be done.