just to avoid future issues with the beloved Regie, it is better to send a letter by registered post notifying that this person is moving in with you. Regies do not proceed with the inscription of the person in your "bail" unless you got married to this person. They just aknowledge their presence in your flat. Therefore they do not care who is paying the bills, as long as the bills are paid. You remain responsible for all obligations.

Reasons why informing regies?  Should you as official tenant be on duty or leisure travel far away from Geneva your (permanent) guest might be meeting disorders in your apartment which needs possible to be fixed by owner or regie and your guest will then be known to regie who will take action for repairs, for instance. Amother possible option is to avoid illegal sublet, like someone gone on a long journey and while away some other person (squatter) gets in the flat and occupy without your permission and communication to regie. Anything happens ? Then YOU are fully liable for any incrued costs. Such cases have been reported already. We all understand the  three letters that summarize, CYA.

Above might be read as contribution to ban future (financial) problems with landlord and YOUR insurance company.