A staff person in my 65 person org recently created an Office 365 group. I wanted to add files to the group, but I wasn't a member of the group. Since I'm an admin, I added my user name to the Office 365 Group and then, for good measure, I made myself an owner of the group.
I see the group now, but I cannot upload any files to the files area of the group. There's no option to upload files or drag and drop them. I can upload files to any other group I created - but I didn't create this one.