A remark posted by a friend in a group conversation puts you on the spot on another subject?

You can answer in private from a group conversation.

To do so: 

1 - Keep your finger on the person's message 

2 - By going to the Options, by choosing Answer privately.

3 - You will then open the existing conversation with this contact, or initiate a new private conversation with this one.    

Source: 01net.com