[sblock=Mechanics]The building was designed with the intent that people would walk on it, so there's a railing. While failure is possible, actually failing badly enough to fall yourselves is... unlikely.
Theoretically I'd say that Kincaid could either make a group check (he also makes a check; at least half of the group must succeed) or help (Caleb gets advantage), but it's irrelevant since 17 looks like a success already.
You're probably also limited to your carrying capacity, but between the two that requirement is met.
For the attempt to actually succeed, though, the bystander also needs to successfully tie herself to the rope. She's unfamiliar with ropes, distracted, and rushing, but she's also bardically inspired and it only has to hold for a few seconds... I think there's a good chance of success there (DC... 12-ish?).
Since Caleb's already made that Strength check I really could adjudicate this now, but I'll wait a bit for a post for Kincaid.
If the players with the characters on the ground want to be prepared in case the ones on the roof fail, to catch the falling person you can either make a Strength check to contribute to the group check or Help one of the others to grant them advantage.
There's actually a DC listed in the book for that one, but it's for 3.5 and assumes the one character making the check gets a slew of +2s from aiding allies so I won't be using it directly.[/sblock]