How to check if a person has joined a group to unlock the service?

Question

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Is it possible to make such an interesting little thing on WordPress.
There is a product and it is free, but I want a person to invite a person to the VK group for this free product (for example, 50 people) and only after inviting these 50 people the service could be purchased by him.
Can this be arranged somehow?

Answer 1

я нагоню тебе в группу 500 ботов и твою группу забанят - и не дай бог ты 10 товаров не дашь.

Answer 2

I propose two scenarios:
1. We read the vk rules regarding provoking users to spam activity, agree with the adequacy of the rules, forget about these nonsense.
2. We create a community application, hang it in the community - a stub.
We give the user a link to this application with an additional get parameter, with which we can understand whose link the person clicked on.
We process the data coming from VK to our application ( https://vk.com/dev/apps_init) and an additional get parameter (I'm not sure here, but now I looked in the access logs of my server - the parameters are displayed in them).
We write in the database the id of the user who came through the link.
We hang a callback on our main group - https://vk.com/dev/callback_api
For a request from a callback with the group_join type, we check against the database whether the id of the new subscriber matches any of those who previously followed the link to our iframe application.
UPD: the application will still need to stick a button / widget for subscribing to the main community.