What role has Abigail Williams been playing in the proceedings?

You do not tell us what point in the proceedings your question refers to, so I will just talk about her role in general.

In some ways, Abigail Williams is driving the whole witch hunt.  Without her, I imagine that there would be much less in the way of trials.

I say this because Abby is the leader of the girls who are at the center of the trials.  They follow her lead in claiming to see spirits, etc.  The best example of this is when Mary Warren comes to claim that the girls have been faking it.  Abby starts to talk about seeing a "bird" and all the girls go along with her.