If you chose not to include any language in your lease that would prohibit the tenant from bringing in a roommate, even as a subtenant, and chose also not to include any provision in the lease that would trigger a rent increase based upon an additional occupant (or occupants), then you don't have a basis to increase the rent.
If this is a month-to-month tenancy, you can increase the rent for future months upon proper notice; otherwise, you may do so at the end of the lease term if the tenant chooses to remain in residence.

