They're allowed to bump it, but there needs to be 8 hours in-between each bump, no matter who bumps it. If it is under 8 hours, you'll most likely be warned/infracted.
Yep, they may. Keep in mind, however, that only one person is allowed to use an account. So if they wish to bump it, they'll need to create their own accounts to do so, respectively.
They are allowed if you state them in first post. That makes you responsible for it. They can bump once per 8 hours, unless some posts require urgent answer.