It's called a collateral meeting. It's done at the patients request. They have to sign a waiver for the other person to attend. They actually appreciate having collateral meetings. Because they can confirm anything that the patient has been saying. The patient has the right to restrict how much information their person is allowed to receive. Or how long they are allowed to participate.
I believe a client can bring someone with them if the person they are bringing is coming in because they want to. The client can't force the person. The client's therapist can't force them either. The person who joins the client will not be the person really getting therapy though- they could come in for a therapy mediated conversation as a way to help the client. Example: you can't bring a friend in who you think needs therapy for therapy with your therapist. This is how I understand this. Maybe this helps?
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