You can add 50 people who are in your friends to the VKontakte conversation. If a person is not in friends, then you can’t add him to a conversation. VKontakte does not provide for such a function. First make the user your friend, send a request, and then it will be possible to add to the conversation.
The answer above is incorrect. Anyone in this conversation can invite new people to a conversation, and it does not matter whether in friends who who created in this way or not. That is, you can ask your friend to add to the conversation of his friend, who is not yours.