Install devise invitable

Devise invitable gem is required to invite users.

config/initializers/devise.rb

@@ -96,6 +96,55 @@ Devise.setup do |config|
   # Setup a pepper to generate the encrypted password.
   # config.pepper = "9a89de1df41b08b48af904bbbc5b8d713addd9d14e2758f12dd14105acd8627fececa02f33f89c0e9764e74262d8874c4a52534b788f5aa4b1716522ae6888b1"
 
+  # ==> Configuration for :invitable
+  # The period the generated invitation token is valid.
+  # After this period, the invited resource won't be able to accept the invitation.
+  # When invite_for is 0 (the default), the invitation won't expire.
+  # config.invite_for = 2.weeks
+
+  # Number of invitations users can send.
+  # - If invitation_limit is nil, there is no limit for invitations, users can
+  # send unlimited invitations, invitation_limit column is not used.
+  # - If invitation_limit is 0, users can't send invitations by default.
+  # - If invitation_limit n > 0, users can send n invitations.
+  # You can change invitation_limit column for some users so they can send more
+  # or less invitations, even with global invitation_limit = 0
+  # Default: nil
+  # config.invitation_limit = 5
+
+  # The key to be used to check existing users when sending an invitation
+  # and the regexp used to test it when validate_on_invite is not set.
+  # config.invite_key = { email: /\A[^@]+@[^@]+\z/ }
+  # config.invite_key = { email: /\A[^@]+@[^@]+\z/, username: nil }
+
+  # Ensure that invited record is valid.
+  # The invitation won't be sent if this check fails.
+  # Default: false
+  # config.validate_on_invite = true
+
+  # Resend invitation if user with invited status is invited again
+  # Default: true
+  # config.resend_invitation = false
+
+  # The class name of the inviting model. If this is nil,
+  # the #invited_by association is declared to be polymorphic.
+  # Default: nil
+  # config.invited_by_class_name = 'User'
+
+  # The foreign key to the inviting model (if invited_by_class_name is set)
+  # Default: :invited_by_id
+  # config.invited_by_foreign_key = :invited_by_id
+
+  # The column name used for counter_cache column. If this is nil,
+  # the #invited_by association is declared without counter_cache.
+  # Default: nil
+  # config.invited_by_counter_cache = :invitations_count
+
+  # Auto-login after the user accepts the invite. If this is false,
+  # the user will need to manually log in after accepting the invite.
+  # Default: true
+  # config.allow_insecure_sign_in_after_accept = false
+
   # ==> Configuration for :confirmable
   # A period that the user is allowed to access the website even without
   # confirming his account. For instance, if set to 2.days, the user will be