You can easily add or remove someone on iMessage. If you want to remove someone from the group text simply go to the group and click on the icons of members. Then click on the info button under the member's icon and remove that person. If you want to leave the group using Mac then open the iMessage and click on the group message. Click on the info icon “i” in the top right corner of the screen. There you will see the option of “leave this conversation” click on it.