If your establishment, to which a new employee holding a PF account has joined, is not covered by the EPF & MP Act because you do not have 20 employees, then the new employee should wait until your establishment is covered by the Act. This means it is not possible to register an establishment just because someone who is an existing member of PF has joined.
However, if you already have 20 or more employees but have not taken the initiative to get your establishment covered, then it is a serious legal issue, and you must take immediate steps to register. Registration or coverage of PF will be retroactive from the date your employee strength reached 20, and you will have to pay both the employees' share and the employer's share from that date, with interest and damages. The interest and damages part can be addressed later, but you must proceed with registration.
Nowadays, at the time of company registration, the EPF and ESI registrations will be done, but only the coverage will be pending. However, for proprietorship and partnership firms, registration can be taken when your number of employees reaches the limit of 20.