That would depend on your design - whether each guest would have their own account and the same account would be used for different events, or a "guest" is just a name and an email address.

Gayaneh wrote:

We didn't consider this in previous design, but as I am thinking, maybe each guest would be identifiable by their e-mail address, if we don't want to add more details for the guests...

Design is up to you. When I grade your homework I'll let you know whether I think it's a good design or not.