No problem. If you are not attending the conference yourself, during the registration process choose "I am registering for this person". This will make you an administrative contact for the entire registration and make it to where you will receive copies of the registration confirmations. At that point, you can use dummy email addresses for everyone else as long as the dummy email address is different for each person. It may not make sense, but the registration system creates a unique registration by way of email address.
If you are going yourself, register yourself as the very first registrant and make sure your email address is valid. This will make you the primary contact for the group. You can then provide dummy email addresses for every subsequent registrant making sure each spot has a unique dummy email address.
You can do this for names as well. Ex.
FirstName1 LastName1 firstname.lastname@example.org
FirstName2 LastName 2 email@example.com
When you finally know who is coming, you can make a substitution request and we can swap in good information. You can do this online, or wait till you show up to the conference and we can do it onsite.
Again, if using any of the above procedures, make sure that at least you as the administrative contact, or you as the first registered attendee in your group, have a valid email address so that you can receive the confirmations.