[hitsthings]

You'd have to ensure they didn't have any privileges, for a start. E.g. you have to ensure you set the role correctly so they can't execute other lambdas in your account, etc.

If you are running multiple users on the same lambda, one could potentially read filesystem data that another saved, for example (if the lambda hasn't restarted).

You also probably want to make sure each user has individual compute limits, so one user can't execute 1000 scripts/second and DDOS other users.

There's probably a lot of little things to think through (that I have not).