Joining the IPMC is just like any other PMC, once enough merit and community involvement is performed, PMC members will take notice and may vote on you to join.
Foundation members may willingly join the IPMC at any time, to express desire send an email to
private.AT.incubator.apache.org
A full list of IPMC members can be found at home.apache.org/phonebook.html?pmc=incubator
The full list of expectations of an IPMC member can be found on the Roles & Responsibilities Page