isolated grounds in branch circuits? How does this work with conduit?
I'll bet what you heard about isolating was about the grounded wire, which is what we normally refer to as the neutral wire. See:
Grounded vs. Grounding 
for a more complete explanation.
Neutral wires indeed need to be kept separate for different circuits in the same box. The exception, sort of, is an MWBC where one neutral actually serves two different hots on a pair of circuit breakers. Mixing neutrals can result in a situation where a neutral gets overloaded because another neutral connection broke, where if they were not mixed that circuit would instead go dead. Plus mixing neutrals will not work with GFCI breakers.
The grounding wires, aka &quot;the ground&quot;, can be mixed without any problem.