BuddyPress.org

Hi @jungmook,
it’s a bit difficult to answer a such wide open question in one post. So very shortly:

For a multisite installation of WP with Buddypress, do all the sites in the network of sites share the same user base?

Yes. There is only one wp_users table in the DB. And by default all registered users are network users.

Can WP multisite with BP be easily configured to work the same way (and yet let each site have its own separate forums and groups)?

Yes and No.
BuddyPress on multisite runs for the whole network. If you’re not comfortable with WordPres and never used BuddyPress it’s not easy at all, because you have to learn a bit first.

If you’re comfortable with WP, it’s relatively easy because BP is “only” a plugin and it works like a plugin.

And if you’ve never worked on a network (meaning a multisite) it could be complicated at first because you probably don’t be familiar with things such as activating a plugin on the main site and not for one of your network site, or vice versa.

I encourage you to read attentively the codex, show some videos about wordpress multisite and get the one or other tutorial you can find on the web.

Some reference

Installing Group and Sitewide Forums

http://www.wpbeginner.com/wp-tutorials/how-to-install-and-setup-wordpress-multisite-network/
http://premium.wpmudev.org/manuals/wpmu-and-buddypress-installation-2/