This sounds like a great idea! It'd be so much fun having piggyback races all around Fluorite :p
It might be a good idea to have it like how you'd add a friend or someone to a group by clicking on their character then clicking on a button saying "Carry/Pick up" A box pops up on thier screen asking them if they want to accept or decline but have it like the menu system that you can only ask them 5 times before it makes you wait, so people cant deliberately spam you over and over again