Hi Elizabeth! That's a great idea! I love running with my family and friends! You will have a great time with your mom and best buddy.
You do not need to register together. They will put you in corrals based on any race times you give them for proof of time. You don't have to give them a time at all. If you wish to be in some of the first corrals, you have to provide a time for a race (a 10K or higher) that shows that you qualify to be in the speedier corrals. This is just for safety reasons and so that the elite runners don't have to swerve around those of us that are not-so-speedy!! (like me!)
Your corral letter will be on your bib when you go to packet pick-up. If for some reason, you are not in the same corral...no worries! You can move back corrals, you just can't move up corrals. So, if you and your mom are placed in "C" corral and your friend was in "D", you and your mom could join her back in the "D" corral.
My only advice is to keep an eye on the
Walt Disney World Half Marathon website. Once it gets past 50 percent capacity, they will keep the percentage posted on there so people will know that it is getting full. It hasn't been posted yet, so there is still plenty of room. It does usually sell out though as the race gets closer.
Please let me know if you have any other runDisney questions!!