Can I add an expense with one friend, outside any group?

Yes, with a friend who is on Evensies. Open them on Friends and tap Add expense, or start an expense anywhere else and pick them under Who is it with?. What you add shows on their page and on Friends as just between you, never under Groups.

It is free while you share a group with them. If you don't, starting it comes with the one-time purchase, as creating a group does; once it is started, it is free for both of you for good.

Starting it needs a connection the first time. After that, adding and settling work offline as anywhere else.

Inside it, everything works as in a group: receipts, comments, history and restoring, repeating expenses, reminders, and settling up from their page together with your groups.

It is only ever the two of you: nobody else can join, and it has no invite link, name, notes, cover photo or settings. It can't be archived, left or deleted.

Export everything as CSV includes it, and a file you import with just the two of you in it can go into it.