Yes, anyone logged in may add tags. Thoughtfully though, as there is no way to remove them yet.
RubyGems.org determines the primary owners of a gem. If a user log's in with the same
email (and verifies it), he will automatically be assigned as owner of his gems. Please
mail us if this is not so.
Additionally, if the gem is hosted on github, persons with commit right to the repository will also be considered owner.
Ranking is calculated by weighing the different attributes and scaling to 100. The exact algorithm is here.