I need to delete a group I created

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.

Add a person

1. Tap the group conversation that you want to add someone to.
2. Tap the top of the group conversation.
3. Tap , then tap Add Contact.
4. Enter the contact information for the person you want to add. Then tap Done.

Remove a person

1. Tap the group conversation that has the contact you want to remove.
2. Tap the top of the group conversation.
3. Tap , then swipe left over the name of the person you want to remove.
4. Tap Remove, then tap Done.

Send a group message on your iPhone, iPad, or iPod touch - Apple ...

https://support.apple.com/en-us/HT202724

It would be better if you can post a screenshot, in order to understand the phenomenon better.

You may get better and prompt responses if you may kindly provide the iPhone Model that you have and the iOS version if it is known to you.