Adding another contact to an existing text messag

How do you add another contact to an existing text message. Online instructions from Apple does not make sense. I specifically asked for the instruction for the XS. Their instructions are to open the conversation and tap the top of the conversation and then tap 'add contact'. There is no 'add contact' indicator on the XS when you open a conversation. So can you add another contact or not? Apple really should review their online instructions because some of them are outdated for all the various phones they have on the market today.