Hi everyone! I have finally figured out the problem.
Even after I had deregistered my phone number from iMessage, other phones were still sending their group messages to me as iMessages, so I couldn't receive them. Here's what I did next:
My network is AT&T. An expert at the AT&T store one day recognized that the IMEI number on my Samsung phone was still listed as belonging to an iPhone (this must have been a relic from when I switched my SIM card from my iPhone to my Samsung). He fixed that for me.
With the IMEI number on my phone now correctly identifying as belonging to a Samsung and not an iPhone, my problem should have been fixed. Other iPhones should have begun to recognize my device as a non-Apple device and their group messages should have stopped sending as iMessages. But even then, I realized that the only group messages that were working were those in which there existed no special iMessage group that had been created between my number and the other phone numbers when I used to have an iPhone. This means that the group had no special name (e.g., "college friends"). When I asked those with the iPhones to remove the name that we had given to our old group chat, their Apple devices stopped being so stubborn about maintaining the group as an iMessage group, and my friends with iPhones saw their arrows and bubbles turn green instead of blue. Their group messages are no longer sending as iMessages, and I can now receive and respond to them freely with my Samsung device.
Hope this helps anyone who has experienced a similar problem with group messaging after switching devices!
