Adding new records based on user recognition

See my responses in bold below. 

So adding a new user to that User Table would have to be done manually by someone correct?  Yes

Perhaps make it a function of whoever does the inviting of users to also add them to the Users Table?  Well yes but totally separately and manually. 

What about adding a User-List field to the Companies Table and selecting every User that is affiliated with that Company. Could we utilize something like that?  No, that won't work.   The issue is that when adding a record the form needs to default the companies field so the user doesn't have to enter it manually. In order to default it in it needs to do a look up from the user stable to see which user is logged in and what their usual company is.