If you already have your form created, and the invite you're sending will ask the leads to fill out that form, then you can use a stand alone smart list with a Filled Out Form trigger. This will show you everyone who filled the form out.
In your Smart Campaign to actually send the invite, your smart list should use filters that will pick out the leads you want to send the invite to. For instance, if you want to send the invite to everyone in your database who works for Marketo, you could use an Email Address filter with the contains option, and then put Marketo in the value box. Or you could use a Last Name filter to send it to everyone in your database with the last name Dishman. It's up to you which filters you want to use.