Relationships

I have a model User who has many Posts I now want a user to be able to group one of his posts with any posts from any user.

so i have a Groups model. in the groups database i have: user_id and post_id

Groups: has_one :user has_one :post

Posts: has_many :groups

so i pull out a post from a user. how can i find what other posts are grouped to that post?