application.yml

User:
  fields:
    default: name, email, user
    form: name, email, user
  relationships: projects, third_party_projects
  application: wide

Project:
  fields:
    default: name, user
    form: name, user
  relationships: collaborators
  application: wide

ProjectCollaborator:
  fields:
    default: user
    form: user, project
  filters: project, user

project.rb

class Project < ActiveRecord::Base
  belongs_to :user

  has_many :project_collaborators, :dependent => :destroy
  has_many :collaborators, :through => :project_collaborators, :source => :user
end

project_collaborator.rb

class ProjectCollaborator < ActiveRecord::Base
  belongs_to :user
  belongs_to :project

  validates_uniqueness_of :user_id, :scope => :project_id
end

user.rb

class User < ActiveRecord::Base
  has_many :projects, :dependent => :destroy
  has_many :third_party_projects, :through => :project_collaborators, :dependent => :destroy
end