Jetstream allocations are done exclusively through the eXtreme Science and Engineering Discovery Environment (XSEDE). XSEDE provides XSEDE User Portal (XUP) accounts free of charge. You can read the Getting Started guide to learn about the process of getting onto XSEDE and applying for allocations and using XSEDE resources.
To review the types of allocations XSEDE and the process to get an allocation, here are some links you might find useful:
Please note, the startup allocation process is fairly simple. It requires a one paragraph description of what you'd like to use Jetstream for and the CV of the applicant/principle investigator. Turnaround is typically one business day.