Two things.
1. ERAS is the application service, not the matching service. NRMP is the matching service. Some fellowships use ERAS, some do not.
2. Your login for ERAS is the same as for residency, but you have to fill out a completely new application. This is because your ERAS login is linked to your AAMC number, which you used for applying to medical school as well. All your demographics have already been verified that way.
OP, I haven't registered for NRMP for fellowship, because it doesn't open till the end of September for my match (all matches this year?), but I imagine they will have some information on file for you, and you just specify which match you are participating in.