I use Enterprise so rarely that I can never remember my Enterprise Plus membership number either (why they don't allow you to set up a user name for login is beyond me). Anyway, I always end up do a search of me email's inbox for "Enterprise" and I can always retrieve my membership number from the initial sign-up confirmation. Barring that, you could always call Enterprise (1-800-264-6350) prior to your trip and have them look up your number and add it to your reservation. Certainly seems easier than doing it at the counter when you want to get your car and leave (and the counter rep wants to move on to the next customer).