The NRLS only applies to tenants named on the tenancy agreement. If two or more people share a property but only one of them is the named tenant under the lease, then the £100 per week threshold applies to that person for the total rent paid.
Where there is more than one tenant the £100 a week threshold applies to each tenant’s share of the rent.
If two joint tenants pay a total of £180 a week rent; their individual £90 a week share falls below the threshold where the NRLS becomes applicable.
If the weekly rent was £250, then each of them would need to separately register with the NRLS and fulfil the requirements.