Why wouldn't you want to alert your fellow hosts to the fact these guests are dishonest and book for 3 but turn up with 7 by writing a honest review of their stay? @Marie1382
Hope you sent them a request to cover the costs of the additional 4 guests.
In terms of hassle - in the time it took you to post here you could have written their review 🙂
In answer to your question, you won't be penalised by Airbnb for not reviewing your guests.