Who to serve the S21 to?

Rent Arrears

Hi, I phoned the advice line and was told that I only need to serve the S21 on the tenant who is named on the agreement and who signed it so I have done this.
However I was reading on the forum that if they can prove that someone else is paying towards the rent then they could claim rights as a Tenant if they wanted.
On my bank account, the rent is payed in from an account which is in joint names my Tenant and her husband. Should I also serve an S21 on him too even though she is the only person on the agreement? I do not want to make out he is seen as a Tenant if it is not needed but then again I do not want him to try and claim Tenancy when it goes to Court as I am sure they will not leave on the 29th December as is required.
Thanks for your help.