The day-to-day operation of the social housing system, including the oversight and practical management of existing tenancies is a matter solely for each local authority.
It is, however, my understanding that persons who wish to move into an existing tenancy may apply to the local authority for the right to reside in a local authority dwelling and to be on to the rent account, for example if the person is in a relationship with an existing tenant. It is a matter for the local authority to assess this application, and approve or refuse it, as appropriate, in line with the assessment criteria it has in place including the control of anti-social behaviour, good estate management, the avoidance of overcrowding, etc.