class Post
  include Mongoid::Document
  has_and_belongs_to_many :tags
end

class Tag
  include Mongoid::Document
  has_and_belongs_to_many :posts
end

Loading development environment (Rails 3.1.3)
ruby-1.9.3-p0 :001 > t = Tag.create
 => #<Tag _id: 4eddc9d5148aaa3328000003, _type: nil, post_ids: []> 
ruby-1.9.3-p0 :002 > t.posts.create
 => #<Post _id: 4eddc9db148aaa3328000004, _type: nil, tag_ids: [BSON::ObjectId('4eddc9d5148aaa3328000003')]> 
ruby-1.9.3-p0 :003 > t
 => #<Tag _id: 4eddc9d5148aaa3328000003, _type: nil, post_ids: [BSON::ObjectId('4eddc9db148aaa3328000004')]> 
ruby-1.9.3-p0 :004 > t.reload
 => #<Tag _id: 4eddc9d5148aaa3328000003, _type: nil, post_ids: [BSON::ObjectId('4eddc9db148aaa3328000004'), BSON::ObjectId('4eddc9db148aaa3328000004')]>