You enter them in the group. Open the group and select Add.
You manage your contacts with a variety of sorts. It's not how many you view
on the screen. You can sort by category, name, company etc.
When you sort by category all of the others disappear off the screen. You
could even create a general category for your current contacts so that when
you select it, your Address Book would look just like it does now but the
business contacts would be there when selected.
Backup! I guess your business contacts aren't that important if you are
asking about that.
Yes, but they are easier to find and edit when in the Address Book. For
example you get an email from a contact, this is a new address. You use
command-option-C to automatically open the contacts old address and add the
new one.
Say John Doe has address
[email protected], but he also has an address
[email protected]. Having all of his addresses in one place makes it easier to
manage.