Connect the grounds from each device to a common point.
e.g. connect the negative leads of all your power supplies to that common point.
Bonding the various components to a common point ensures all of the signals are referenced to the same “ground.” That can help keep serial datacomm (as well as other electronic devices) from acting strange.
Grounding, or earthing, is for safety. Given the low voltages involved in an emonTx and RaPi,
there’s not much of a safety hazard. Nevertheless, it’s not a bad idea to connect the common point
to earth to keep static charge buildup to a minimum.