In my planed multisite installation, I want the members to be able to post in one of two different categories.
Members in A can post in 1. Members from B can post in 2. All members can comment everything.
Then, I plan to create a custom page template to the blogs index, and I hope to query all posts from members in 1 and 2, and display title and a minor excerpt (along with avatar image) in separate columns (widgets).
Is this possible? (I am not planning to use Buddypress).
For restricting the categories, maybe the plugin Restrict Categories can help. But it doesn’t have Multisite capabilities.
Take a look at the filter list_terms_exclusions that it uses in the cat_filter function.
But Role Scoper states that it has “Extensive WP-mu support”. May be worth a try.
For the query part, take a look at this Q&A, find a random blogid across my multisite network that has at least one post published.
Basically, you need the functions switch_to_blog and restore_current_blog.
