Members area only - limit access

Hi there, need some suggestions please.

Running a Worpdress site with a forum plugin. What would you say is the best plugin that will:

-Allow access to search engines like Goolgle.
-Request a standard "password" to access any of the pages.

Its basically a training resource site and we want to have one password that everyone can use to access the site.

Tried some plugins, but they either not compatible or they don't do what we want it to do.