I haven't set a radio group, how does the micro:bit allocate group numbers?
The micro:bit runtime code (the DAL) puts a group code in the first byte of all payloads transmitted via the radio interface. At the receiving end, if this group code does not match the group code set by the receiving script, the payload is ignored by the receiver, and your script will never see that data payload.
The group number can be any number from 0 to 255. The group number at the transmitting end, and receiving end, must match, if you are to transfer data between two or more micro:bits.
The radio interface on the micro:bit sends broadcast messages, so if multiple micro:bits have the same group code set then they will all be able to see the messages sent by others. It is also possible to change the group code while the micro:bit is running your code, so you could use different group codes for different purposes, on a single micro:bit.