Hi Lara. You can automatically add users to specific group(s) inside your community if the user has a specific differentiator on his/her account.
The differentiator can be something like: user's payment plan on your website, country, language, role, or any other information that you have from your side on the user's accounts.
Zapier is much simpler, but if you decide to use the webhooks, the Webhook you need to use is "user.signup".
If you are worried about removing users from those groups when the user is not eligible (for example the user is not on your paid plan anymore), you have 2 ways to automatically remove the person from the group:
_Use an API to update the paid member's list (this should be done by your tech team from your end).
_Or to use the Stripes's webhook (or any other payment software's webhook that you are suing) to remove the user from a group on Tribe if the payment is canceled.