How to save user-related data
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I recently bumped into 2 projects (with wordpress) which require to save data into a users profile.

First of all the users have to register.
Secondly the users will click on a article or any other post type to visit it.
Next the user want's more of this article so he decides to spend money for it.

Now here's the tricky part: I haven't worked with user-related data until now, so I need you. Where to hook in when I want to save a buy or for example a click on a button which makes a user guest on a party's attendees list.

Thanks in advance for your help.

Cheers!