[r-a-y]

It depends on how integrated you want your site to be.

If existing members can’t login to the main techsentry.org site, then I’d just install BP on the blogs.techsentry.org subdomain WPMU setup.

However, be aware that a BP setup under the “blogs” subdomain will have permalinks like:

eg. http://blogs.techsentry.org/members/USERNAME

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#2 would work. I’m currently migrating a standard WP site to WPMU for the sole purpose of using BP and user blogs. If you have to alter a few settings to get things right, but your permalinks would stay the same.

#2 is the hardest to implement, but the benefits are single sign-on from one point of access.

It all depends on your needs.

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Not really related, but as a usability thing… you should preferably list your blogs when people land on the blogs.techsentry.org subdomain homepage.