CPR 19.4 states the following with regards to adding and substituting parties once a claim has begun:
(1) The court’s permission is required to remove, add or substitute a party, unless the claim form has not been served.
(2) An application for permission under paragraph (1) may be made by –
(a) an existing party; or
(b) a person who wishes to become a party.
(3) An application for an order under rule 19.2(4) (substitution of a new party where existing party’s interest or liability has passed) –
(a) may be made without notice; and
(b) must be supported by evidence.
(4) Nobody may be added or substituted as a claimant unless –
(a) he has given his consent in writing; and
(b) that consent has been filed with the court.
This means that an application must be made to add or substitute a party. No party can be added as a claimant unless they consent in writing. This makes sense of course – it would be strange if someone could add someone as a Claimant unless that person actually wanted to litigate.