You will have to use the email connector, and then if you want to send a single user or multiple users the link to invite them,
Now for the registration form, you can create a registration object and send its GUID (A java action from community commons) into the mail and then use deeplink or page link (from page properties) to send its link into the body of the email.
This way you can send the mail to a user,
If you wish to send it to multiple users, just make a list of all users and then use an iterator to send them emails separately.
Let me know if you face any issues.
You can refer to the image below,
Hope it helps!