Yes, your landlord cannot restrict guests without good reason, or charge any fees for your guests, even if they are staying overnight. However, always remember that when it comes to guests, you have to be reasonable. If a guest stays over for an unreasonable length of time, your landlord may consider them an additional occupant and try to raise your rent or evict you.
When you invite guests on to your landlord’s property, you are legally responsible for their behaviour. You have to make sure that your guests are not violating other tenants’ quiet enjoyment, not causing damage to the property, and not engaging in illegal activity. If your guests cause these types of problems, your landlord may be able to restrict their access. If you continually allow your guests to cause problems, your landlord may issue you a warning letter and/or eviction notice.