Creating Custom Promotions & validating profile fields

[vibethemes]

I new new to VB and have been trying to setup a support form.
My requirement:
For each purchase I give user a purchase code which she uses to gain access for the support forum for that respoective product only.

What I have dont so far?
->I have created usergroups for each product, where each product forum is accessible to that user group.
-> Added profile fields for users to enter purchase code against each product.

What I need to do and How can I do this?
-> Validate purchase code using my http API on Profile field saving.
-> Add additional usergroup to the user's groups if validation is successful.

The second one I believe could be done by creating some custom promotion to change the usergroup.
Please suggest if you've a better idea to how can I achieve this...