Re: Existing Theme, Buddypress Features
The sitewide activity, members, and groups are all widgets which would simply needed to be added to your sidebar. Other than that its as simple as installing BuddyPress and your main site will be maintained while the profiles will assume the new theme.
If you’d like to see a more custom example, check out MMAFrenzy.com where I integrated the profiles into my own design but for basic functionality and maintaining your own theme its just a basic install.
Hope that helps, feel free to ask any questions so I can hopefully clear them up for you.