Yes of course, and your housemates do not need to join CreditLadder for you to join. Sign up, let us read the rent from your bank and you will gain the benefit.
If only one person pays the landlord/agent then only they can benefit. For you all to benefit you each need to pay your own shares of the rent directly to the landlord/agent. Then each sign up for your own CreditLadder accounts.
If you would like all of your flatmates to join all please register separately. We take individual accounts to ensure we can allocate credit reporting separately.