Tenants' overnight visitors

Tenancy Types and Management

1. When my lodger first rented a room, he said he had no girlfriend and only wanted a room in which to sleep and study not to socialise. Now he is bringing his girlfriend to stay the night once or twice a week every week. If she stays 3-4 nights a week, it would be like two people living in my home. Legally can I tell him she can’t stay the night? I prefer more quiet rather than charge more rent.
2. A tenant renting a room in one of my rental flats is letting her overseas visitors stay the night every few months. Sometimes 4-5 people stay for several nights in her room. Another time her parents stayed for three weeks in her room. Other tenants complained. Can I ask her not to let friends and family stay the night? Can I charge rent to these overseas visitors sleeping in her room?
3. Three girls are renting a whole flat from me and every time I go round to inspect the property, at least one of their boyfriends is staying the night there. I usually turn a blind eye to it since they rent the whole flat. But if all their boyfriends are staying 3-4 nights a week regularly, is that considered as 6 people living there? Do I have to get an HMO licence? How do you prevent 3 people renting a flat then 6 people end up living there?