There appears to be no way to "add to contacts" from a group message if there are unknown numbers in the recipient list. For a single message, the option appears under the drop-down menu. That option does not appear for a conversation with multiple recipients. Drilling into "Conversation details" from that menu shows the unknown numbers clearly under the heading "Members", but tapping/pressing them gives no option to "add to contacts" or even to copy/paste the number. It just makes the keypress sound and does nothing. Touching a known contact shows them in the contacts app.
There appears to be no way to "add to contacts" from a group message if there are unknown numbers in the recipient list. For a single message, the option appears under the drop-down menu. That option does not appear for a conversation with multiple recipients. Drilling into "Conversation details" from that menu shows the unknown numbers clearly under the heading "Members", but tapping/pressing them gives no option to "add to contacts" or even to copy/paste the number. It just makes the keypress sound and does nothing. Touching a known contact shows them in the contacts app.
Running version 5.19.3 on Android 8.1.0