Just to add my 2 cents as well, from the sounds of it you need an open place with parking and water, how about the Elora Flats (Wilsons Flats) just outside of Inverhaugh? There's also a park in Elora along the Grand with open space to practice casting as well as a park in Fergus along the Grand that has some open space for casting.
The river is full of public access locations so everyone could team up and try out different locations after the meet up. And afterwards there's pubs in Elora and Fergus to have a pint and tell fish stories afterwards
The link
http://www.grandrivertroutfitters.com/river/ has public access locations. It's not like the Grand is a secret either it's renowned for its brown trout fishery, and would be able to handle the pressure of a one day outing scattered from Fergus/ Elora/ Inverhaugh etc.
Hope this helps and I would also be interested in coming out.