Imagine you're an event planner...the traditional, terrestrial kind.
Would you consider hosting a trade show booth, networking luncheon, cocktail reception, or seminar at a hotel somewhere without considering all the details, tangible and intangible, that are part of the experience? Would you invite people to a "great presentation" but not tell them where to park or how to find the restroom?
Don't do it for a web seminar either.
Design the user experience every click of the way from the first thing they see when they get an invite to the last follow up afterward.
Then test it.
Walk in their shoes.