The USA Rugby site has links to Rugby clubs (ie Rugby Union) across the US including Denver. "Ruby League" is a different game that split from Rugby over 100 years ago. There are RL competitions in the US but you are far more likely to find a local Rugby (Union) competition.
First, get your games right. There is rugby union and rugby league. They are two separate sports. There is the Denver Barbarians, Denver Rugby Club and there may be a new one, Denver Harlequins. These are all rugby union clubs. There is no rugby league in Denver. These clubs are all on the net.