First, you should be adding youtube_it to your Gemfile, then running bundle install.
This gives bundler enough information to figure out the dependency graph of all the gems your application needs. Bundler writes out the needed gems, their dependencies and versions to the Gemfile.lock file.
Second, when running the rails command, you shouldn't need to run bundle exec according to Yehuda Katz (http://yehudakatz.com/2011/05/30/gem-versioning-and-bundler-doing-it-right/). However, in my experience there have been times when bundle exec has been necessary even for the rails command.
In any case, running "bundle exec executable" will run your executable (rails, rake, etc) with the gems listed in the Gemfile.lock, avoiding the dreaded "can't activate -- already activated" error.