Under the Residential Tenancies Act it is not illegal for a tenant to have temporary or even permanent guests as long as the tenant is still living in the unit. Unfortunately the landlord has no recourse in this situation even if the tenant is collecting rent from these other people. You can only do something if your tenant is not paying you the rent or they are causing damages to the property in which case you would serve the appropriate notices and proceed to the Landlord and Tenant Board to terminate the tenancy.