No one can stop you, but accounts are free. There’s no reason to share. Also, it will be impossible to track your progress separately from the people you share the account with.
There's no real good way to do that, as your account stores your study data for individual terms and schedules limit you to studying only what you need to each day, so it can only handle a single user. Having multiple people use the same account would prevent you fron studying what you need to practice and make it difficult to track what you know vs what the other user knows.
This place isn't a game site if that's what you're thinking. It's not built for that. This is a place of learning. It's ok if it's someone like a tutor trying to guide you through things, but that's not necessary. You are in control. It's your knowledge being tested, not someone else's.
Having someone else mess with your account is a sure way to get yourself lost, so that would be very unwise. You can reset things, but it's not worth the headache.
To oversimplify it, it would be like having a personal trainer for two people who are both working on different things, but the trainer can only remember one training regiment, so both of you get a mix of what you want/need and the other person, with no way to correct the trainer and tell them what you can/cannot do, what you do not want to do anymore, etc.
