Hi there : ).
The Appointments+ plugin is really great, but in my opinion it's missing a key feature.
You've included the ability to limit the number of spaces in a given time slot (if you have 10 service providers but you only want 2 people to be able to request an appointment in each time slot, that's doable). That's great.
However, you have not provided a way for one service provider to allow multiple individuals to reserve the same time slot. With that ability, this plugin would be great for a wider variety of users--for example, life coaches and fitness coaches who offer group coaching (one service provider with several open appointments in the same time slot).
Without this feature, unless I'm missing something, the plugin is kind of useless for anyone who wants to provide any kind of group appointment (life coaches; physical trainers; group counselors for churches, marriage counseling, job training; etc.).
I did see this in the FAQs (http://screencast.com/t/pYXDZd4kQeW7), but neither of these solutions seems practical to me in a multisite setting. If you add the code to your theme's functions.php file, you change the setting for all sites using that theme across your installation. Adding the code as a custom plugin that you activate only on sites where you want to change this functionality might work, but only if you want the potential group sizes to be the same for all sites. Otherwise you need a new custom plugin for every site that needs this addressed. And, maybe I'm just not seeing how it works, but I tried creating multiple dummy providers using the same WP user and it only allowed me to create one dummy per WP user. If you could create 15 dummy providers and assign them all to the same real WP user, that would solve this issue, allowing you to have 15 openings for one time slot. But as long as you have to create a new WP user for each dummy provider (with a unique email address), it's not a practical solution either. Am I missing something when it comes to the dummy providers option that would solve this problem?
I would love it if you would add in this functionality...would make the plugin so much more useful to a wider audience.