Add basic invitations

app/models/invitation.rb

@@ -0,0 +1,14 @@
+class Invitation < ApplicationRecord
+  # Relations
+  belongs_to :user
+
+  validates_presence_of :user
+
+  before_create :generate_token
+
+  private
+
+  def generate_token
+    self.token = SecureRandom.hex(8)
+  end
+end