Hi Paul,
I see no one has answered. I don't use a 20D. You 50mm lens is great for lots of things but not the best for small heads! 50mm is about the focal length for getting in a 10 people 10ft table at a wedding from about ten feet or so with a FF camera. So for a 20D your zoom lens at about 35mm or less will be better.
When is this shoot?
I'd set this up first somewhere else and practice. 2-3 rows will be needed.
If you just go wide in one row, you are going to end up with tiny heads!
Instead, can you find a place with risers or stairs which allow the people to be arranged so that the taller people can be behind and you can get everyone in.
You may need 6-8 chairs for the front row. You can then space the heads of one row between the heads of the next row. If you have one accomplice, then it is easy to rehearse with just 4 pics with that person in different spots to check they are will illuminated. If you test, as you must, you'll do fine!
A large white card to the right and left may be sufficient to bounce back the light to people at the sides.
You are going to have to check for fall off. Still with only 6-8 people per row and just 2-3 rows you should be O.K.
Don't forget to use a WhiBal card in the center and at the end of the rows. Use you flash meter to check that there is not too much drop off from the center to the sides. Pulling the flash back, will decrease the intensity of the illumination to the group. However, it will be more uniform. There will be less lateral fall off and the light readings will be close in the 2-3 rows. The low white ceilng will help, of course.
Asher