Modulators on a group channel should be able to modulate anything on channels that are grouped by the group channel.
For example:
I have 4 different synths that I want to use the exact same modulation values for filter cutoff. Instead of copying and pasting the curve 3 times, just record the modulation once on the group channel and apply it to all the synths in the group.
This is how Bitwig modulators work.