must install BuddyPress, in a new installation of wordpress.org absolutely?
You can only install BuddyPress in a self-hosted WordPress blog, i.e. WordPress.org
required wordpress.com account for this to WordPress.org?
multi-users functionality of WordPress 3.1 is similar to multi-blogs(multi-sites)? may install in one site and in another run other blog, same wordpress installation or needed a new installation?
== required wordpress.com account for this to WordPress.org? ==
No, WP.com account is not required to install WordPress. You should know this as I’ve seen some of your posts at WP.org
=== multi-users functionality of WordPress 3.1 is similar to multi-blogs(multi-sites)? ===
Not exactly. A single WP site can have multiple users. When you https://codex.wordpress.org/Create_A_Network, you can create many sites with one WP installation. [edit] And each site in the network can have a single user or multiple users, it all depends on the set up.
For more information, post in Multisite forum at WP.org – https://wordpress.org/support/topic/welcome-to-the-multisite-forum?replies=2
Good luck.
I think lselwd is getting confused at some of my replies where I stated setting up multisite gives you your own *version* of wordpress.com. It works like that, but it not it. 
single user or multiple users
with this you mean roles/permissions/users?
may install in one site of multi-site wordpress, only? better install in wordpress only one site, or it does not matter?
Go multisite (create a network) and install BuddyPress if you want to allow your users to create a blog/site of their own in your installation. Otherwise, do not create a network, stay single WP installation.
you say: stay single WP installation,
but other than my main blog, brand new install? hence my main blog if for it purpose…
I do not want users create a blog/site of their own.
=== but other than my main blog, brand new install? ===
It all depends on a variety of factors which include what your realistic expectations about adding BuddyPress are. I don’t know what your current site/blog is all about so only you can determine if social networking will enhance your current site’s branding or if it would be better if the social networking feature was a separate entity/site altogether.
