iMessage group how I can add a person on the group

Hello someone could help me to tell me how to add a person in iMessage group since i can not find the position on the iPhone 7plus does not bring the option or i have to activate it someplace this phone please help since it had to form several Different groups to add more people here I leave an example in the photo that I do not appear the position thank you for your time and help!

Note:

The first picture is my phone

And the second picture is the examples I found looking for others iPhones