Hopefully you got this sorted, but jumping in with some best practice / inspo for webinar contacts here.
Regarding consent for marketing communication:
As @karstenkoehler indicated, this depends on your region. If you are in EU and under GDPR, you should get explicit consent by the registrants to do any marketing communication outside of the webinar. Typically, you would add a consent box in your signup form whether you (A) run registration via HS form, or (B) via the webinar software directly. But always consult with a data protection or legal expert here, as this is no legal advice.
Best approach to sign webinar attendees up via HubSpot
I recorded a YT video on how to work with a webinar platform together with HubSpot to sign people up here - these flows should work no matter if you work with a HubSpot Enterprise plan or a Starter/free license.
But to summarize in text form below.
1. HS forms vs. webinar software registration page: I would recommend to use HubSpot forms if you have workflows available and need flexibility of having registration on your own website - like adding additional tracking. If you need things to be as simple as possible, just use the webinar software. Make sure it supports adding a checkbox for marketing consent/privacy policy here though. Univid is an example of a webinar software with native integration (top rated on g2 and HS marketplace) that supports all of the flows I explain here. Typically, you work with a workflow after form submission that registers the user. But, you could also use a hidden form field, if the webinar tool supports it - and you are on a smaller HubSpot plan.
2. Sending reminders:
For most hosts/marketers I would recommend to rely on the webinar software here, and use their built in reminders, typically you can customize a few things in the reminder emails. And most importantly, the automatic emails already include the personal link + add to calendar buttons (meaning you can track people without additional logins).
If you are pro/enterprise - and need all the details in the emails to be customized - you can typically access the personal links from HubSpot though (in Univid you use a custom action for this) and then also get the add to calendar links. But that would naturally require a plan that supports workflows and marketing emails in HubSpot.
Hope this was some good inspo on using HubSpot and a webinar tool together to sign people up and stay compliant - let me know if you have any questions. And good luck! 