I just tested it and found that a group registration of two members only records the first member - the second member is not recorded.
In the Records list (in the administrator), it's recorded as a group registration and the Members column shows "1 members".
On closer examination, it seems that when you add a member to the form, you must click "Add Member", which is completely unintuitive because a new blank form is presented. When you "Add Member", it's being added to the left column and the user is supposed to leave that form blank and scroll up to click "Complete Group". This is going to cause a TON of problems with users who think that when they fill in the form and click "Complete Group" that the second person is being recorded. The "Complete Group" button belongs to the right of the "Add Member" button at the bottom and its action needs to both record what was entered on the current form page as a member and go to to the next (payment) step. THAT would clean up this interface.
In addition, if I try to use the same email address in a group registration for each group member (often done in my client's use case), it prevents me from using it, saying: "Invalid field: Email". The user (registrant) is NOT logged in.
I can't install this version on my client's sites because it's going to screw up a lot of people who think they're registering a group, but will come up one short in their registrations because they assume when they fill in the form an click "Complete Registration" that it's recording that form as well - but it isn't.
I tested CSV Export and see that Item Price, Amount Paid and Confirmation Number fields did not populate in the export.
I'm still on 4.0.9 (with patches applied) to make DT Register work for my clients - the 4.2.1 interface is greatly streamlined and more attractive, but if people are not getting registered properly, I can't use it.
If support need admin acccount access please see https://www.joomlart.com/forums/d/23086-dt-register-4-2-0-hangs-on-payment-page-after-patch-installed.
Thank you,
Ernie