You are allowed visitors. However, you are as responsible for their actions as if you did it. If they damage the walls, you are responsible for repairing those damages. If they smoke while in the building, this will constitute as if you smoked in the building. In essence, you are held responsible for their actions. If the visitor spends a couple of nights every so often, this does not constitute as them living there. If they are receiving mail, then not adding the visitor to your lease will be a violation of the lease. You are required to report changes of household promptly.