Gmail – Manage G Suite for Client

Since mail@[...] was taken and inaccessible to me, I just registered mail2@[...] at first.

Then I had access to the admin console, I could call Google support and that's when I managed to get mail@[...] back and remove the temporarily created other address.

This is a bit of a workaround, but most importantly: I was able to call Google support. I didn't need to, because from this point, all I had to do is:

• Add the original mail@[...] account to the super administrator group
• Remove the old one

From the comments, to provide an accepted answer: If you wish to use the classic folders, give them a link to the folder you wish them to access, adding ?usp=sharing to the end of the link. It will look something like https://drive.google.com/a/mydomain.com/folderview?id=0B7FYz‌​rvwemtv0EyM2VBVDA&us‌​p=sharing. When they open the link, in the top right should be a button to "Add to Drive". They then click that and the drive will show in their My Drive. Note that I put this link on an intranet page for all new users to visit. That page has instructions for this and other items unique to the organization.