Redmine

projects = Project.where(id: [1])

prefix = 'testuser'

5000.times do |i|

  login = sprintf("#{prefix}%05d", i)

  user = User.create!(:mail => "#{login}@example.com", :login => login, :firstname => login, :lastname => login)

  projects.each do |p|

    Member.create!(:principal => user, :project => p, :role_ids => [1])

  end

end