Spruce Brook shelter has a couple of tent platforms up on the hillside, and in a pinch you could fit a tent or two in the flat area behind the shelter. Because of the slope, I think you'd have to go further in or out along the Wild River Trail to find more campsites. There's a composting privy there in fine condition. Filter your water right out of the river. I can't predict popularity; the Saturday night I spent there last August had 4 people in two tents, and me.
Perkins Notch shelter (picture
here has a huge area for camping off to the right (when facing the shelter). You could fit probably a dozen tents back there and still be comfortable. See
here and
here . In July of 2002, the privy was just a pit toilet in somewhat
sorry condition and many just went into the woods. Do note that not too far beyond the last tent area, you will come to the Wild River Trail, which has curved back on itself. Oops.
While you're there, do go back on the WR Trail from the left of the shelter, then watch for a small footpath on the left (west). Follow it and step carefully and you'll come out into an
open area with
views of Carter Dome and of Perkins Notch and
No-Ketchum Pond .