Justin, having not been properly established at this point, currently has the power of "asks convenient questions".
Susan, meanwhile, is demonstrating her extablished power of being the smartiest smarty pants of all the smarty pants.
Grace is also demonstrating a power in this comic, that being the power to not remember something until just the right narrative moment. Most characters in all forms of media have that power, but it's super important nonetheless.
Ellen demonstrates no powers whatsoever in this comic. That's just something that randomly happens to people from time to time.