well-respected by Goodman Brown, as well as the nice old lady who taught Brown his catechism, Goody Cloyse, are all part of the congregation: they're all sinners, because they all have a natural "instinct that guides mortal man to evil"—according to the narrator of the story:

[Brown] fancied he could distinguish the accents of towns-people of his own, men and women, both pious and ungodly, many of whom he had met at the communion table, and had seen others rioting at the tavern.

Even Brown himself, so eager to remain faithful and to resist the devil, gives in, becomes a "demoniac" who rushes toward the witch-meeting, "giving vent to an inspiration of horrid blasphemy."

So, Brown, along with the members of his community, truly are communing in the forest, in a ceremony that resembles a church communion service.

As the other Educator has pointed out, the witch-meeting resembles a church communion meeting in that the mood is somber, the music is sung together, and the people focus on their sins.

I'll also add that, much like in a church communion service, this witch-meeting is presided over by a leader, a "sable form," who commands the group's respect and wears official clothing:

With reverence be it spoken, the figure bore no slight similitude, both in garb and manner, to some grave divine of the New England churches.

And, both a communion meeting and this witch-meeting are symbolic of mankind's union, of the things that knit us together and make us one. The "sable form" declares,

Evil is the nature of mankind. Evil must be your only happiness. Welcome again, my children, to the communion of your race.

However, it also makes sense to compare this witch-meeting to a baptism service rather than a communion service. "Converts" are presented to the entire group, and they are ceremonially marked or touched as a means of initiating them into the congregation:

Herein did the shape of evil dip his hand and prepare to lay the mark of baptism upon their foreheads, that they might be partakers of the mystery of sin, more conscious of the secret guilt of others, both in deed and thought, than they could now be of their own.

This is a story about Young Goodman Brown losing his faith in goodness and in humanity. Brown adheres to an old Puritanical religious fundamentalism. In such a belief system, moral and ethical codes are very strict. When Brown starts on his journey, he is full of hope and faith. He believes that people are good or bad, no in between. But as he sees the church elders and supposedly pious people of his town consorting with the devil and attending a witch-meeting, he loses faith in humanity. Brown can not accept that good people can be capable of sin. 

Brown will not accept that supposedly good people would consort with criminals and disreputable people. One could interpret this story as a very pious man realizing the presence of evil in the world. But the story is also a subtle criticism of strict religious beliefs which instill in someone like Brown the idea that a person is either wholly good or wholly evil. This is a very "black and white" view of the world and is impractical and unfairly judgmental.

In recognizing that anyone is capable of evil (as much as they are capable of good), Brown loses his faith in humanity. Rather than accepting the fact that people are fallible, Brown rejects humanity. 

When Brown loses his faith in humanity, he also loses his faith in Faith. After seeing saints and sinners together, and his wife Faith in the midst of all this "deviltry," Brown gives up his belief in the power of goodness and acknowledges that "Evil is the nature of mankind." 

"My Faith is gone!" cried he, after one stupefied moment. "There is no good on earth; and sin is but a name. Come, devil! for to thee is the world given."