How to send email invite user fill in webform automatically?
There are a handful of different modules that can let you do bulk emailing (e.g. VBO, Mass Contact, Email Campaigns, etc.), but using Drupal as a batch email client I think only make sense in the context of:


Having regularly visiting user base, with pre-existing accounts.
Needing to contact a relatively small number of people (e.g. hundreds, or very low-thousands). 


Your 4th requirement suggests that you'll be supplying any number arbitrary emails for the sake of feedback/lead gathering.

In lieu of a module or Rules (which only make sense in the context of some workflow built into the site), I'd suggest some 3rd-party email service (e.g. Constant Contact, MailChimp, etc.) to accomplish items #1-4.
Any decent 3rd party email service should give you a templating system with substitutable tokens that will let you customize your webform link if you need it to have pre-filled data (e.g. email, name, attended seminar, etc.) and a means to bulk submit email/contact info.  

As far as item #5, there's a number of different way to accomplish this. One way is the Forward module which provides a share via email link/form for any node type.