Is it appropriate to require registration?
It depends for what. Imagine what a doctor’s office would look like if people did not have appointments. The staff would not have any idea what kind of service they would need to provide that day and how many hours would be required. Besides making it impossible for the medical staff to schedule their day, they would not be able to prepare adequately to tend to the needs of the patients. Additionally, it would cause hardship on the patients because the waiting time would be unpredictable.
A conference could not be arranged without registration. The organizers need to know for how many people they have to prepare accommodations and programming. Without that information they might lose a considerable amount of money if they prepare for many while only a few show up. If they are careful and cautious and limit their spending, and surprisingly there is a great turnout, they might not have sufficient provisions to provide for the participants. Although having a large turnout is great, it could leave everyone upset and frustrated if the organizers are not adequately prepared.
Then there are special events. Some individuals habitually don’t respond regarding their plans to attend. This can leave the hosts in a quandary. The host wants to present an enjoyable event. The host may be pressured from the caterer for a number which is needed for ordering and set up. The host, while waiting for responses, does not know how to answer or what to do.
Regarding the Korban Pesach, the Paschal lamb offering, the Torah states that it should be taken, “According to the number of people.” The Talmud states: “This teaches that the Pesach may be slaughtered only for those registered to it. It might be thought that if he slaughtered it for those not registered, he should be considered merely like one who transgressed a commandment, but the offering is nevertheless valid after the fact; the Torah therefore states, ‘According to the number ,’ and then again says, ‘Shall you count.’ The verse repeated the requirement of registration to make it clear that it is essential.”
Why is registration for the Korban Pesach mandated and non-negotiable?
The requirement of registration will motivate everyone to take action to prepare and participate. No one would be able to figure, someone else will take care of it and I will show up. People will have to not only get involved but make the effort of joining a group. The Korban Pesach is too important a matter to be left for someone to say if I can make it, I’ll be there. A commitment is required.
Do we refrain from or delay making commitments or do we enthusiastically respond, “I’m in!”?
Rabbi Hershel D. Becker
 Boh 12:4
 Pesachim 61a
 Rambam Moreh Nevuchim 3:46