Hi Stefanie,
Do you have access to use Inquisium Surveys? I would think the best way to acheive the result you are looking for would be to collect the save the date responses in the form of a short survey.
Otherwise, I would agree with Gema (above), that an Express event would work. The benefit there being that people can quickly accept or deny the save the date, so you know who is interested, and also who would prefer not to receive further emails (ie, the early bird invitation, etc.). The downside would be the added registrations used up in your account.Â
If this is a very large event, or an event that will repeat in the future, I would look into Inquisium. If it is on the smaller side or a one-time meeting, I would do an Express event.
I hope that is helpful!