Get list of users.

I'm not sure this is generally possible without the use of a plug-in.  Can you share what use case you have that requires you to not use any plug-ins?

There's actually an already existing group called "All Users" that does exactly what Stefan said.  It uses a rule to gather every user in the system to itself.

I don't know if this still holds true, but in the past users were flagged inactive by being placed in an "Inactive Users" group that you could find, though I can't seem to find it now.

Handy little snippet for finding all of your group IDs:

a!forEach(
items: enumerate(300),
expression: try(
concat(fv!item, " - ", group(fv!item, "groupName")),
"That's not a group"
)
)

You shouldn't try using try() for anything that ever might make it to production, but for your own investigation in /rule that you're going to throw away in minutes, I say knock yourself out.