I don't have any knowledge of the Hendrie Valley specifically, but what you saw are almost certainly Great Egrets. They are becoming increasingly common in Ontario. The only place I've seen that many together is in the Leamington / Wheatley area.
The only other white egrets that you might find are Snowy & Cattle. Both are uncommon in Ontario, so the chances of seeing 5 or 6 together would be very remote.
BB