Multi-user seems to have been something which was dropped when MS/DOS was CP/M inspired by CP/M. CP/M (and I do mean CP/M, not just MP/M) had the concept of user areas. These were a different concept from Unix users: there were 16 user areas numbered 0..15, and one could change between them with the USER command. I believe that user area 0 was in some sense shared, possibly only as a place to load .COM files from. There was no security boundary: it was just an organisational tool.