If it's free for all users, and you don't provide any benefit to those "tipping", it's already an untaxed gift in the US, if no individual gifts more than $19,0

If it's free for all users, and you don't provide any benefit to those "tipping", it's already an untaxed gift in the US, if no individual gifts more than $19,000, and even then, the gift giver would pay any taxes. Tips require a customer relationship to exist.