It may a decent small money maker, albeit with a certain element of risk.
Depending on what county you live in, probably the worst thing to happen would be you'll get a warning for piracy. From one of your users downloading pirated software, music, or video.
Absolute worse cases could be very bad, such as threats made to government officials or other people, or child porn. But I think those are very unlikely to emanate from your small group of users.
You'll also want to keep in mind any data caps that your plan may have. If your plan has any caps at all, unless you have a foolproof way to limit each user's data (your router software may have that capability), you should not even consider trying to share your connection.