When you already have a key/secret for the Events API, you can use the same for the Users API. It's easy as that. Otherwise you should apply for a key and update your credentials on the Culturefeed API settings page.
Good to know: our user authentication platform is also known as "UiTiD"
For some specific purposes you might want to enable Culturefeed Search Views, Culturefeed Entry UI, Culturefeed Saved Searches, Culturefeed Calendar, Culturefeed Pages or the Culturefeed Messages modules. Attention: these features are still in beta and not fully supported in the Culturefeed Bootstrap theme.
If you want to extend some functionalities or build your own modules, you will need the UiTiD API documentation (only in dutch).
Culturefeed Bootstrap has some basic theme settings where you can customize branding colors, font-family, border-radius, navbars, etc. to your own needs. If you are a Drupal themer you can - of course - change all possible less variables and bootstrap overrides or kick everything out and build your own theme :-)
If you like the Culturefeed Bootstrap theme or plan to build your own Bootstrap subtheme some knowledge about the Bootstrap Framework (version 3.0.2) will come in handy. Good to know: Culturefeed Bootstrap uses Less to compile the CSS.