I've allowed PCs to carry each other, both with and without flight as part of the equation. Earlier editions used to limit how much a creature could carry and still fly, but 5E doesn't have anything like that. Because I like to have flying monsters periodically grapple and try to fly off with PCs, I'm fine with this. One of my players is currently taking advantage of this, with his new gnome wizard having a vulture as his flying mount (he even spent most of his starting wealth to buy an exotic saddle for it).