Capacity issue

This plugin is great and I thank you for that.
We're a coworking space and i want to use it as a timetable for coworkers. They need to be able to show to other coworkers when they plan to come over or to see wh'o coming.

My first issue is that the "capacity" for the unique service I've got doesn't work. I enter "0", which means "no limit" and after having entered an appointment, if someone else want an appointment a the same time too, it doesn't work.

Is it possible, when i click on a free time slot to choose the end time?

Thanks for your help,