I have already spoken to Kaylee on this issue before so I think I'll state some things:
Technically, she does get her own say in who can ride with her since it is her car. She doesn't even have to carpool anyone, let alone do a free one, and it really is out of her generosity, so really, I suggest not making the same mistake I did and complaining about her carpool in specific.
However I would also like to note that you have more voice power in this than you say Gage. Mooch has told me that there are a few times that he was able to get a seat just by asking you to ask Kaylee. You may not directly control who gets a ride, but when you ask for her to pick someone up, there's a higher chance she'll accept your request than decline and take someone else.
This isn't really your fault since it's still her choice in who will get that privilege (and it's also not something to get mad at her about, for reasons said above), but I sometimes think the choice in seating can be silly, notably when it's someone who has their own car like Alex or Erik (I don't know if Erik's ever ridden with her, but I'm highly sure Alex has carpooled with her before), when they could've held a second carpool and made whatever tournament more available to more people. Also, do I even need to mention that one occurrence with Brick?
/Drama llama