Organizing community surveillance against ICE to warn locals when agents are in the area? What are the legal bounds on this? Would it work? Would it help?

Hypothetically, I imagine community group chats, whose members also text warnings to other locals not in the chat, and those people then spread the word further. When agents are seen lurking around town, someone sends the word out, and maybe people can even gather in person and follow the agents to warn passersby, film any arrests, remind people of their rights, and protest.

What's legal and what's not?