Drop a member from a group message on iPhone, or work around the limits on Android and mixed groups.
On iPhone, open the Messages app and tap the group thread. Removal only works in iMessage groups (blue bubbles) with four or more people — you can't remove anyone from a three-person group.
Tap the group icons or name at the top of the conversation to open the group details screen, then expand the member list if it's collapsed.
Swipe left across the member's name and tap Remove, then confirm. The group gets a note that the person was removed, so everyone will see it happened.
Mixed groups with Android users (green bubbles) and standard Android SMS groups don't support removing members. Create a new group text with everyone except that person, and let the old thread go quiet.
If the goal is fewer interruptions rather than removal, use Hide Alerts on iPhone (group details screen) or the notification mute option in Google Messages on Android to silence the thread without changing its members.