Ok I'm going to steer the pot a bit
I could see the Carrot sneaking on the ship thing as a possibility, much like with Bon Clay back in Alabasta
Carrots absence may be due to her going to make sure the sunny is OK and made it off of Onigashima safely
And with the Luffy's quick acceptance on letting her stay after Pedro was willing to turn the ship around and take her back to Zou, I could see her sneaking on board and the asking to stay
And just like Yamato asking to join and being denied because Luffy didn't trust her but was quick to trust her with Momo and post Wano when she asks to join again and Luffy accepts because of the shift in his understanding of her and their new found trust
After this arc has concluded and Carrot is found on board, having snuck on again
Her relationship with the crew could lead to them having no objection to let her stay
Plus we've already seen that Wanda (her big sister type) give Carrot her blessing in leaving so long as she is with someone she trusts
Last time it was Pedro and now it with the saviors of their country, an allied country and friends
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STILL DONT THINK SHE'LL JOIN
But I won't discount the possibility of the scenario given the past events