The simples thing to do is to buy enough leis for all the guest, but why not make a game out of it?! Instead of getting made leis or making them yourself, just get the supplies and let the guests make their own.
You can then have a vote on who made the best lei and deal out a few Hawaiian theme prizes (like a hula skirt or a really pretty lei).
You could also give out a little lei making class before the start of the party, or make it an optional activity during the party. There are many YouTube videos on how to make them, so feel free to check them out. Personally I like this one(because it's simple and it uses silk flowers, which make for a permanent lei that guests can take home as a souvenir):