How to make A+ login with a register option & Multiple person appointments

1. I have some clients who don't use social media at all. I will want them to be able to register and login to use the calendar/SMS system I have set up. I've looked in settings and don't see a way to make a registration screen for them. Where do I make the registration screen for those without social media?

2. I'm using the calendar more like a daily pill reminder than an appointment calendar. How do I allow more than one person to select the same time? I'm currently using 10 minute appointments.
I saw a post about changing my function.php file. I must have done it wrong since it didn't allow more than one appt afterwards. What is the best way to do this?

Thanks!