Hi Vincent,
Great choice, you'll have a fine time.
Trail sneakers will be okay–Emigrant travel is relatively easy for the Sierra. I'll recommend debris gaiters, as there's a lot of commercial horse packing and the main trails leading into the interior are wide, deep and dusty so it's good to keep it out of your shoes as best you can.
Overnight lows in August are unlikely to be lower than the mid-20s and in more typical conditions will stay above freezing. Rain is possible, so plan on bringing some rain gear. Chances are it will stay stowed, but storms can slip in from the east, occasionally. It's generally in the 60s and 70s during the day.
Unless we have gobs of snow in the next month, it will be dry and pretty much bug free by August except perhaps by marshy lakes and meadows. (Famous last words–check trip reports to verify, in case there's a wet spring.) On that note, small streams will be dry by then, so consider that when planning how much water to pack for the day's hike.
Cell coverage is very unlikely once you're away from the highways. A messaging system like SPOT is a dependable way to check in and the folks at home can even check your progress. Highly recommend something like that for your group.
Cheers,
Rick