how to add another person to a group text

Add or remove someone from a group conversation

Anyone in a group iMessage can add or remove someone from the conversation. You can remove a person from a group iMessage that has at least three other people. 

You can’t add or remove people from group MMS messages or group SMS messages.

This answer doesn’t tell us how to add someone just that we can. This is not helpful at all as the question was how not if we could. Why would this be the reply to the question ask? The only thing I can think of is the person answering ignored the question or doesn’t understand English.