If you go to https://www.fitnessfirst.de/club/api/checkins/muenchen2, you’ll get a JSON object with max allowed and currently checked-in people at your Fitness First studio.
You just need to replace the club name with your own club and you’re good to go. Just go to your home club page with any browser, and look at the source.
Search for: <section v-cloak class="show-club-checkin" data-club="muenchen2">
and you’ll find your club name.
For putting the data in a sensor, I use:
Open Developer Console (F12) and search for data-club below block-clubcheckinsblock. You’ll need to copy that number from data-club (e.g. 1376532610)
Alternatively, reload the page with open network tab and you should see a request with a number (e.g. 1376532610).
Create your custom sensor (e.g. in configuration.yaml) with the following yaml: