Update User List field using actions or automations

Wouldn't that only work with some set number of users? My workflow here is that whenever a record is modified in a certain way, I want the record owner of that record to be added to the user list. 

The Limit appears to be 10 users.
https://login.quickbase.com/db/6ewwzuuj?a=dr&r=fx&rl=dy6

But it is smart enough not to duplicate users.  Do you need the result to be a List User field type?  Or can it be simple text?

I do think it needs to be a user list type. The field is primarily used in an automatic notification I have set up that emails each user in the list. 

You would need to make a formula which would detect if a semi colon delimited string of email addresses contains the email address of [Last Modified by], and if not then add it to the list.

Then you would need to have say 20 or so fields to parse out all those email addresses so you could use them in a notification.  A Notification would need to have separate fields.

So bypass the user list function altogether? I guess that could work. But then how do I automatically trigger that formula? I suppose it could be a button or something if need be. 

I have not thought through the whole formula, but the function User() is the current user and hence UserToEmail(User()) will be who will be the [Last Modified by] when the record is saved.

Ok. I'll see what I can do with these ideas. Thanks for all your help, I really appreciate it.