First of all, private chat with others requires the consent of the other party. You must first add the other person as a friend, and the other person needs to accept your Friend Request before you can chat with the other person privately in the QQ group.
Secondly, according to the regulations of Tencent QQ, the number of private chat messages sent to non-friends every day is limited. If you continuously send private chat messages to multiple non-friends, it may trigger the anti-spam mechanism of Tencent QQ, resulting in your account being blocked or restricted from sending messages.
In addition, administrators in QQ group can also set some permissions to restrict the private chat behavior among group members. Administrators can set the prohibition of private chat, supervise private chat, and only allow private chat between group owners and administrators.
It should be noted that the above information is based on the current understanding of the answer, because the policies and regulations of Tencent QQ may change over time. For specific restrictions and requirements, I suggest you consult Tencent QQ official documents or QQ official customer service to obtain the most accurate information.