There's some challenging requirements around this. You need to positively identify someone as having the right and ability to vote (no dead people voting like in Illinois). You need to make sure they vote only once (no ballot box stuffing). You need to make sure their vote wasn't coerced (as far as you can). You need to make sure their vote is anonymous (to protect the voters from retaliation). And it needs to be easy enough so that your 80 year old grandmother can do it.