Anyone in the UK can join and its free to register. You do not need any proof of address, immigration ID, or a NHS number.
They are a place to identify health issues and usually are the first point of contact if you are feeling unwell or have a health problem. Its easy to register through the NHS website where you can find a GP local to you. It is not necessary to have any form of ID but it can help if you have a Passport, Birth Certificate or a HC2 certificate.
To register you can either contact the surgery by call or email, or see if you can register online via the NHS website. You maybe required to fill out a GMS1 (General Medical Practitioner) registration form. This form can be found via GOV.Uk website.