They'd probably be the ones running the group, or a like-group. One simple tactic is to just pose as your enemy online, which is quite easy to do when it is a digitally created persona/group and not in-person.

The Russians proved how easy that is. Putin has been doing this internally for a very long time before messing with the 2016 election. Create a pro-gay group, create an anti-gay group. Foster hate between them. Others join the respective "causes" that they align with. You now know a lot of the pro/con supporters because you're running the group(s).

Here's an 11 minute clip from Adam Curtis' HyperNormalization, which illustrates this tactic: https://www.youtube.com/watch?v=kwglZTNhMNE

It's alive and well in the USA too, and things make more sense after watching it. I'd find the whole video and watch it. It's quite fascinating.