With devise 2.1.2 and devise_invitable 1.1.8, the page you enter after setting the password from the invitation link depends on the fact that you set the root path for your development resource in config / routes.rb, so @trh answer doesn't works with this version (I tried and failed). From the development code comments:
# The default url to be used after signing in. This is used by all Devise
In my case, I had two different namespaces with one namespace called "portal", which I used for this class "PortalUser". For Rails 3.2, I simply declared this line on my .rb routes:
get "portal" => 'portal/dashboard#index', as: :portal_user_root
An important note is the naming "portal_user_root", which is the underlined name of the class name PortalUser. Just setting this named route is all that your devise_invitable needs to redirect as desired.
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