In your members area, go to the "Advanced Features" page, then click on the "Misc" tab, then put the url for your webinar page in the "Redirect Page" field.
Put the full url, including http://.
Also, put ?user=~username~ at the end of the url so that it will go to the appropriate member's page.
Yes, you can modify the signup page to have less fields. All of the fields are required, but you can replace them with hidden fields and give them a default value (exa: You can make a hidden field for address and give it the default value of "111 Main St" and then after they've joined they can go back and change it). The minimum fields you can have on the join page is:
Technically, name is not required (but we do not recommend removing it). Also, technically you could give them a default password, but we do not recommend that either.