Ah yes. cf_dev2's comment sort of highlighted for me what was
wrong with your code. You missed the quotes from your evaluate()
statement. And as cf_dev2 pointed out, it's actually preferrable to
use a direct structure/key reference, instead of evaluate.
Nevertheless, you could do this in one of 4 ways as I see it:
<cfset guests = evaluate( "form.guests#i#" )>
<cfset guests = evaluate( "form.guests"&i )>
<cfset guests = form["guests#i#"] >
<cfset guests = form["guests"&i] >
I personally prefer the last one.