How can I count posts in last month by a specific group of users?

[Mittineague]

I think a JOIN with the group_users table ON user_id WHERE group_id should do it.

group_users
id    serial    primary key 
group_id    integer    fkey groups 
user_id    integer    fkey users 
created_at    timestamp 
updated_at    timestamp 
owner    boolean    default false 
notification_level    integer    default 2