In short: You can add 'em before and/or after. The "beneficiary upon death" doesn't need to submit any documentation, that's the only beneficiary that doesn't, but any "responsible persons" or persons listed as "lifetime beneficiaries" that have access to and/or "can enjoy benefits of the trust" have to be submitted. Trustees and grantors always have to be submitted. The beneficiary upon death doesn't have to be listed because upon death that person will have to setup their own trust or otherwise maintain the existing one and become grantor/trustee and thus a responsible person.
So what if your "lifetime beneficiary" has access and is a responsible person but you don't want to go through the hassle of submitting them? Revoke his benefits, then you can add that person back later. Technically a trustee can also be treated in this manner by a grantor but if you do that time and again I'm sure the ATF will frown upon it. ALL grantors will have to submit documents no matter what.
IME, it's best to have one grantor and that grantor also be the only trustee. Otherwise you'll wind up like me, get a surprise divorce and be forced to remove them later. If they refuse to sign off on it, it can be trouble and a real hassle. Luckily mine signed off and restating the trust was pretty simple then. If she weren't a grantor, I could have removed her myself.
41F changed some things and it may be a good idea to talk to your attorney and see if there are any updates or changes that need be made. Good luck!