BuddyPress.org

Sure they can post on single WP. You just have to upgrade their default role from “subscriber” to at least a “contributor” (needs your approval before post is published) or an “author” (can publish right away) and they’ll post from the backend (wp-admin Post > Add New).

Or, not change their roles and use Simple Front End Post plugin http://buddydev.com/plugins/bp-simple-front-end-post/

And/Or, just allow them to post in Group’s blog using this plugin http://buddydev.com/buddypress/blog-categories-for-groups-plugin/