Q: txting on iphone 6s plus- can we add or delete a person from a group

Did you created the group?

If not, you can remove yourself > Leave a group. Tap Details, then tap Leave this Conversation.

Add someone to a group. While viewing the conversation, tap Details, then tap Add Contact. The person you add doesn’t see messages sent within the group prior to you adding him or her.

Grdn1 wrote:

 

yes I created a group, sent the message and realized I forgot one person. going to groups MMS then detail doesn'st give any option to add or remove.

Send a group message with your iPhone, iPad, or iPod touch - Apple Support

You can only add/remove a person from a group message if all users in that group are iphone/ipad Apple product users.  You can't do it if there is a non Apple device in the group - that is why you're not seeing those options in details.