Jennifer:
Yes, admin can sign-up people from the front-end. In fact, any one who has access to that form can sign-up any body.
However, if admin, or any other user, is logged in when they sign someone else up, the sign-up will get associated with the person who is logged in (WP user ID is saved in the signup table). That would be fine if the people don’t have their own accounts and you aren’t allowing them to clear their own signups. If, however, you want the signup associated with somebody else’s account (so they can see their signups in the list and clear themselves), then the person doing the signing-up should be logged OUT of the site when doing the sign-up. In that case, my plugin will try to find a WordPress user match from the entered email, and associate that signup with that user (if a match is found). If you are hiding sign-up sheets from the public, you may need to temporarily allow public to see them while signing up other users so that you can sign-up others while logged out of the site.
As far as entering/editing signups from the admin side, there is nothing in the plugin to do that at this point, and no current plans on adding any new features at this time unless some people want to chip in to cover the development time. Signups can already be entered by admin, or anyone else, as described above. If you make a mistake, you can clear the signup and re-enter it, or use PHP MyAdmin to edit the info in the signup table. Sure, it’s not idea, and would maybe be nice to have admin be able to add/edit from the backend, but this plugin started its life as another plugin which didn’t have those features, and it wasn’t something our PTA chair people needed, so I didn’t bother to add that.