Fort Collins has a "three unrelated" ordinance which means that no more than three people who are unrelated can live together in Fort Collins. In most cases, if a rental unit is occupied by more than three people, they must all be related to each other. There is an exception for owner-occupied dwelling units if the owner obtains a Home Occupation License from the City.
In general, the City does not enforce the three unrelated ordinance unless someone, usually a neighbor, complains. It is very risky for tenants to violate the three unrelated ordinance, because the tenants as well as the landlord can be fined. In addition, the City can require the tenants to come into compliance, which means that one or more of the tenants would have to move out. In most cases, the tenants would still have to pay the full amount of rent, and therefore the tenants would suffer a significant financial loss.
Remember, unless you are all related, no more than three roommates should live together. It is inadvisable to violate the ordinance. If , knowing this, you decide to violate the ordinance despite the risk, you should negotiate a clause in your lease saying that, in the event that one or more roommates is required to vacate due to the three unrelated ordinance, rent will be reduced proportionately. Also, remember that the City must prove that you are more than three people living together, and that you are not related.
For more information about the three unrelated ordinance, take a look at the three unrelated ordinance publication provided by the City of Fort Collins.