Apple Messages has two group chats: iMessage groups for Apple users and SMS/MMS groups for all users.
You cannot leave an SMS/MMS group because it uses your carriers messaging service.
you might only quit an iMessage group if everyone uses an Apple rig with iMessage enabled.
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You cannot leave the group if one or more users do not use an Apple unit.
Why is it unavailable to leave a group chat on iPhone?
You cannot do this if a group chat uses Message at the cellular operators rates.
You must upgrade your iOS to the latest version to leave the chat.
Also, ensure you do not have the Do Not Disturb function activated.
To leave the chat, you must turn off Do Not Disturb.
Otherwise, the Leave this Conversation button will be grayed out.
How to remove someone from a group text message?
How to mute SMS group chats on iPhone?
To unmute the group conversation, follow the same steps and turn off the Hide Alerts switch.
How to add someone to a group text message?
Note that the contact must have an iPhone and use iMessage to be added to an iMessage group conversation.
