One of the marketing newsletters I get is from iMedia Connection. In today's email blast, they feature an article talking about how emails should be brief and to the point, with the main pitch point and call to action "above the fold" (this is an old newpaper writers' term and refers to telling the main story summary in your first paragraph, above the fold line of the newspaper - everything else is just supportive detail). The article states that people spend an average of 15-20 seconds per opened email. That average may be correct, but I'll bet it's WAY overstated for mass marketing blasts. People spend longer on notes from friends and the latest joke of the day, leaving scant time for your invitation to a webinar.
Your invitation email should state right up at the top the subject of your webinar (which is more important than its formal title), the date and time it will be broadcast, ONE reason it is of value to the reader, and a clear request for them to register, along with an easy to use click through. Everything after that is just supportive detail.
In a similar vein, when you send the login instructions for registrants, make sure the email starts out with a clear statement that these are important instructions for listening to the event they already registered for. Tell the reader to keep them in a safe place for reference on event day. If you start with a recap of the event title, subject, and description, your readers are likely to discard the message, thinking it is another marketing solicitation.
The practice holds true, by the way, for introducing content in your webinar. Tell your audience the important thing up front, then substantiate it with details. Far too many web presentations start with an introduction to the speaker's company, where it is located, how long it has been in business, how many employees it has, and so on. Your audience has not decided that they are interested in you yet. Give them the main message, support it, and then when you have them hooked, supply them with comforting facts that let them know they can do business with you in confidence.