Multiple transmitters\receivers all trying to talk to each other on the same channel\frequency is a problem common to most all RF devices, its not a LoRa issue as such.
You probably need to implement some form of addressing, say a byte or two at the beginning of the transmitted packet identifying the source and or destination for the packet.
With a lot of transmitters sending data to a single receiver there will inevitably be some collisions of packets so if you need reliable delivery the transmitter should be using a send and acknowledge system so it knows the single receiver has actually had the packet it sent.
One way around the collision issue is for the 'receiver' to send requests to each of the transmitters in turn for them to return thier data, but that does then mean that the 'transmitters' need to spend a lot of time powered up and listening.
Then there is the issue of the receiver needing to be sure the packets it has received are from one of the transmitters it expects and not from some rogue interloper. Again here you can use markers or identities in the packets so the receiver can identify 'geniune' packets.