This topic has been discussed at length. Take a look at the links below.
https://forum.trustedhousesitters.com/t/do-you-allow-sitters-to-have-other-guests-stay-in-your-home/
https://forum.trustedhousesitters.com/t/asking-for-guests-to-stay-over-on-longer-sits/
https://forum.trustedhousesitters.com/t/pet-sitters-asking-for-guests-to-stay-as-well/
I approach this on a case by case basis. If an owner states something in their listing that would make me think it not suitable for a visitor, i.e. their home and/or pets are only suited to a solo sitter or something about their pet’s temperament, I do not ask.
In the few instances, when I have asked to have friends stop over for a visit I have told the pet owner all about the friend, their education, job, interests, how I met them, how long I’ve known them, their experience with pets, etc. and emphasize that I completely understand if they would rather I not have a visitor and that I am very happy to comply with their wishes.
If they consent, I send a picture of the friend after they have arrived, usually with the pet. I have never had a pet owner refuse but I have asked infrequently and only if the situation felt right.
I have had my daughter sleep over with me at several sits, she is not mentioned in my profile. Again, I told the owners all about her and sent pictures. I also had a friend spend the night at two sits, this was actually offered by one of the pet owners, as they were formerly international sitters and had enjoyed having their own family members stay with them at sits.
I have sometimes asked before the sit is confirmed but sometimes after, as well. If afterwards, I begin the request expecting they will not want a visitor and double and triple check that they have no reservations before proceeding.