Landlord has to register the rental with the PRTB, and even for month-to-month a certain number of things have to be agreed: ideally in writing.
Also, if one of the tenants, probably the case here, is the only one with a contract with the landlord who they pay, and then sub-lets the rooms to sub-tenants, s/he has to have written approval from the original landlord, and technically s/he is under the same obligations as landlords themselves.
"If you sign a lease together with other people, you each become responsible for all the rent. So, if your fellow tenants cannot pay their share of the rent, you may be legally liable for the entire amount. If you sign a lease on your own on behalf of the other tenants, you become responsible for all the rent."
Notice to terminate the tenancy is 4 weeks (28 days) for the first 6 months, and increases with time.
So if you pay your first rent on the 1st of the month, and want to leave on the 5th, you still owed the next month's rent. You are OK if you give notice on the 2nd after paying the current month in full.
Termination still needs to be given in writing. (Not verbally, by phone call, by email or by text...)
More interestingly, it seems that in the absence of a lease (or when a fixed-term lease has expired), the tenants does not have to wait for 6 month before acquiring "Part 4 rights and privileges".
Which can be paradoxical if the tenant who sublets without a sub-tenancy agreement has not secured such rights himself: he could be sued by both his landlord and his sub-tenant!
(This is why the Act has as section called "Effect of assignment of Part 4 tenancy.")