Which type of tenancy do i create for these occupants.....? (Part 2)

Tenancy Types and Management

Hi. I have a 3 bedroom residential property. Some tenants (workers + students) would like to rent the property and they seem sensible enough to make good tenants. The setup here is: 2 couples. 1 x full time worker and his student girlfriend & again 1 x full time worker and his student girlfriend. I don't believe that this qualifies as a HMO setup. The main worker has agreed to take on responsibility to ensure that the rent is paid for the duration of the contract (each month), regardless of the comings and goings of the other 3 tenants. Can i create just one contract with one of the full time workers for this type of scenario for all the tenants? Or do i need to create a contract with each of the 2 full time workers.

On a side note, am i responsible for council tax if the contract holders is/are workers?

Thanks