Nope. There needs to be some sort of opt-in mechanism to make it “legal” on DreamHost. You can feed in all 5,000 addresses, but they’ll all end up getting invitations. And that might be the only step they need to take (accepting the invitation). I’ve not tried this approach, but it’s the minimum that will happen. Maybe someone knows if there will be a second confirmation, but I doubt it. A confirmed email invitation should be enough.