The internet has a number of descriptions of the circuit. Here are a few details that are missed (and important for groups).
Maps of the Powell Lakes circuit can be found online and paper copies are at the visitor center in Sechelt and Powell River
The prevailing wind on Powell Lake is an inflow wind, so it makes the most sense to do the circuit clockwise if you have weaker paddlers. The downside is you have to do the portage #19 the hard way (~200m elevation gain over 1.5 km, then drop down to Windsor lake. Expect to take a whole day to do this portage.
School groups often split into two groups, half paddles clockwise and leave boats at #20 (2-day paddle for youth), while the others paddler the other way and leave boats at #18 (3-day paddle for youth), then swap boats and avoid hauling boats over the hard portage.
The campsites are in great shape with lots of picnic tables, some bear cache and bear hangs, and pit toilets.
The 'large' campsites that can fit 4 or more tents are #20 (the largest), #18 (7 tent pads), #15 (~7 tent pads and room for overflow), #5 (6 pads and room for overflow), and #2. #8 could also accommodate a group but it's not ideal. I don't know about #29 or the fliddlehead cabin sites. The remote access sites are more peaceful than the vehicle access sites.
You can park a car long-term at site #1. However, the last km is quite rough. If you are just dropping off or picking up, and not leaving a vehicle, then it's quite easy to do that at the Lois Lake boat launch at the north end of Dixon Rd (where it turns into 'Goat Mainline')
If you're short on time then start at the north end of Windsor Lake and paddle south.
The Ireland Lake (insert 'C') is easier than the Little Horseshoe Lake portage route, although longer.
Wheels would be useful on portage #3 and most of #9. Otherwise the trails are too narrow, and rough.
When heading north on Lois Lake, expect to take extra time navigating to find the pull-out for portage #3. It's on the Horseshoe river and when the reserovir is low then you have to walk the boats up the river ~500-700m. Finding the river mouth is also tricky due to all the dead wood in the reservoir.