If a tenant is unable to meet the income requirements of a property or fails referencing, they can list a guarantor. This is someone who is not a tenant but agrees to pay the rent if the tenant/s are unable to.
In most circumstances, guarantors don't have any involvement in the tenancy once they have signed their guarantor agreement. If, however, the tenant/s end up in rent arrears, the guarantor will be responsible for this if the tenant does not resolve it themselves.
All tenants are jointly and severally liable for the rent payments, meaning if there are multiple tenants, all tenants are ultimately responsible for ensuring all the rent is paid.
If one tenant is continuing not to pay their 'part' of the rent, the guarantor would ultimately be responsible for this, regardless of the tenant they are a guarantor for.