It does not appear that the “Warehouse” contains a user list any longer. It must have been taken out in some previous version. That usually means there is another method to do this. but like you, I have not yet found it.
Probably. However, when you open that pandoras box in permissions, you are then going to have to supply user level permissions for everyone and for every task they may need permission to access.
It is hard enough to manage group level permissions without adding another level of complexity that is several times more intense.
If you have not started with user level permissions (at least in v10) from the beginning, then it will be significantly more difficult to set it up after you have already established much of your system without them. There always seems to be some process that was working before that you now have to revisit to figure out how to get the user permissions correct in order to get it working again. It can be done, but do you have the time to devote to it without impeding your regular day to day business?