The following will join the Users table to one row in the Donations by using a sub query. This will ensure that at least 1 row exists in the Donations table but will prevent multiple rows per user if they have more than just 1 donation. This will also return the most recent donation.
--join on sub query, ensure at least one donation exists and get most recent donation
JOIN Donations MostRecentDonation ON MostRecentDonation.Id =
SELECT TOP 1 Donations.Id
WHERE Donations.UserId = Users.Id
ORDER BY Donations.Id DESC