TSQL join, sum, count

I have a sql statement that I'm using and am trying to add one additional column that counts GuestRegID from table Guest, joined by StudentID.  The problem is that when I add it, the SUM(CPSTransaction.AuthorizationAmount) doubles the value when there is more than 1 guest record.

This add's the sum correctly.

Select Registration.RegistrationID2, Registration.FirstName, Registration.LastName, SUM(CPSTransaction.AuthorizationAmount) as TotalAmountPaid
FROM Registration LEFT JOIN CPSTransaction ON Registration.STUDENTID = ApplicationIDPrimary 
WHERE Registration.IsActive = 'True' AND (CommunicationPreference = 'Email') AND (CommsSent='0') 
GROUP BY Registration.RegistrationID2, Registration.FirstName, Registration.LastName

Select allOpen in new window

When I add the join and COUNT(Guest.GuestRegID) AS Guests it doubles the value.

Select Registration.RegistrationID2, Registration.FirstName, Registration.LastName, SUM(CPSTransaction.AuthorizationAmount) as TotalAmountPaid, COUNT(Guest.GuestRegID) AS Guests
FROM Registration LEFT JOIN CPSTransaction ON Registration.STUDENTID = ApplicationIDPrimary LEFT JOIN Guest ON CPSTransaction.ApplicationIDPrimary = Guest.StudentID
WHERE Registration.IsActive = 'True' AND (CommunicationPreference = 'Email') AND (CommsSent='0') 
GROUP BY Registration.RegistrationID2, Registration.FirstName, Registration.LastName

Select allOpen in new window

For each Registration.RegistrationID2, I only want to show once, and want to show the count of the GuestRegID from the Guest table joined by Student ID.
I've tried several different scenarios but am needing some help.  What am I missing?
Thank you.