Facebook – Show all posts and comments in a Facebook group that I am a member of, not an admin

I haven't tested this, but since the new lists feature, you are able to share your statuses with different groups of people.

When I share something, how do I choose who can see it?

Before you post, look at the audience selector. Use the dropdown menu to choose who you want to share a post with.

...

Custom (Includes specific groups, friend lists or people you’ve specified to include or exclude)

Taken from this help page.

So,

1. Create your list
2. Go to Privacy settings
3. Choose Custom as your default option
4. Choose Friends from the Make this visible to drop-down list
5. Insert the above created list in the Hide this from field
6. Click Save Setting

Now the default visibility for your posts will be Friends without the persons in the above created list (Custom), so they will not see your usual posts.

1. To include them also, click on the "audience selector" button (the one at the left of Post) and choose Friends
2. To post only for them, click on the "audience selector" button (the one at the left of Post) and:
   1. Click on See all lists...
   2. Click on the respective list(s)

However, please note that the persons in the respective group will see each other's names if they click on the post's sharing icon.

Go to https://www.facebook.com/USERNAME/allactivity or click on the activity log on your profile page, and from there you should be able to view all your group posts (you can filter by category of activity), and then you can delete the comment.