Generally only those with a full time paid faculty appointment can serve as Principal Investigator on a sponsored award. Consult the JHU Faculty Gold Book for information on faculty appointments.
For guidance on Postdoctoral Fellow eligibility to serve as PI, please consult the Postdoctoral PI FAQ here: https://ora.jhmi.edu/faqs/can-a-postdoctoral-fellow-serve-as-pi-on-a-sponsored-project/