Choosing a GNU/Linux distribution is quite a topic (and a moving
playing field!) but I'd start with a highly trusted source of
information...
http://www.linux.org/article/view/selecting-a-linux-distribution
The initial problem facing an individual will be down to their own
combination of what they want to put it on (hardware: exotic?
interesting? locked-down?) and what they want to achieve with it
(software: suitable replacement packages available? generic?
up-to-date? popular and well used = well supported).
If in doubt - just try Ubuntu: massively used and supported, easy
try-out CDs and installers, still meets the criteria of Free Software
and based on Debian so a solid package management system with plenty
of downstream/upstream support.
That's all I would put into a generic GNU/Linux workshop because
everything else would need to be tailored to the needs of the
individual attendees and their pet computers and in most of those
cases I'd just be a human google interface!