This requires a 3 step zap.
1. First trigger: The trigger of the Zap is probably the payment confirmation and it contains the email, name and preferably the username of your customer.
2. Create User: The first action is to create the user. In this case, you will be defining a password and the user is required to reset the password to be able to access the community. (You can use Webhooks by Zapier App)
3. Add Group Member: Since the ID of the created user is available on the second step, you can just add a custom value for User ID and use the information on the second step to add the user to the specific group. (You can use Webhooks by Zapier App)
Here's a link to the Zap wherein we are using Stripe as a payment provider.
Zap Link- https://zapier.com/shared/3f98312176060d5ceeef1e2426d4c1666ab122ee3
Note:- We are working on including "Create User" as a step in Tribe App within Zapier and when it's available, I will add a note here. In the meanwhile, the above steps and Zap can be a good workaround as long as you have the premium version of Zapier.