The issues surrounding this question has always been a source of mystery for me, though there is a definitive answer to your question. In the first act of The Crucible by Arthur Miller, we meet Betty Parris as she is lying on the bed, unresponsive to anything around her. Her father, Reverend Parris, is at her bedside weeping, though as we learn more about him we understand that his tears are as much for himself and his reputation as they are for his daughter.
We also discover that Betty and some of the other girls in town, including Betty's cousin Abigail Williams, were in the forest dancing (and doing other things) last night. Abigail is the one who finally admits that to her uncle, but that is only because she knows he saw them in the forest. After she tells him people are waiting downstairs to talk to him, he asks her:
And what shall I say to them? That my daughter and my niece I discovered dancing like heathen in the forest?
When he asks her more specifically what they did in the forest, Abigail makes her confession:
We did dance, uncle, and when you leaped out of the bush so suddenly, Betty was frightened and then she fainted. And there’s the whole of it.
Parris is embarrassed, but he is also afraid and angry, so he tells Abigail what he saw:
I saw Tituba waving her arms over the fire when I came on you. Why was she doing that? And I heard a screeching and gibberish coming from her mouth. She were swaying like a dumb beast over that fire! I cannot blink what I saw, Abigail, for my enemies will not blink it. I saw a dress lying on the grass.... And I thought I saw--someone naked running through the trees!
So, Parris is the one who saw the girls in the forest last night. He saw them dancing, saw Tituba waving her arms as if she were casting a spell. and saw one of the naked girls running into the forest.
What is puzzling about this is, of course, what Parris was doing in the woods last night. Though he may have followed them to the forest, he was clearly not there to stop Abigail and Betty from doing whatever he thought they might be doing--or he would have stopped it. Jumping out at them from the bushes implies that he was somehow spying on the girls, though he apparently does not see enough (or perhaps wait long enough) to know whether any actual witchcraft is being done. And the fact that he does not react out of moral indignation and outrage at such proceedings implies that he was less concerned about the spiritual ramifications of these events than he was about his own reputation and standing in the community.
In all, Parris's presence in the forest last night is quite perplexing; however, if he had not been there and seen as much as he did, Abigail may never have confessed as much as she did about last night's activities in the forest.