I'd be interested in joining a GB trip in August, preferably later or earlier though. I'm a level 2 kayaker and am familiar with most of the East and North shore of the bay, and so can help with the issue of route planning etc.
In my experience, the bay is a little busy with guided trips in July and August with their trips taking over the available over-night group camping locations, particularly those with thunder boxes. There is also extended power boat traffic during that period. The camping situation can be ameliorated somewhat by working around the trip schedules of the major guiding organisations in the area, including White Squall, Black Feather, Wolf Den, Wilderness Adventures, Voyageurquest, to name a few, but during those 2 months there will likely be sharing required with other groups, so the wilderness experience is not as present as it would be in the off-season. The boating traffic creates issues at the put-in/out as well as when the planned route crosses navigation lanes in/out of the major bays.
For the off-season, September is ideal, the water is still warm the kids are back in school so the bay has quieted down, and the bugs are gone. Late spring (say June) is beautiful, and the area is empty, although bugs are an issue for the uninitiated, and the water is still cold, so proper gear is mandatory. June provides a beautiful wilderness experience without the otherwise long drive to more northern locations.
ps. Going forward, I'm also looking around for a paddling buddy for 5+ day tripping in GB and elsewhere, if that's something you'd be interested in discussing, we can talk about that further.