All baristas should familiarize themselves with the NLRB and what they are allowed to do. You can freely talk/advocate etc any information about a union as long as it doesn't reasonably interfere with work. Remember you are at work, you still have a job to do. This would mean you can't take over DTO and talk about unions to everyone while in between orders etc. While cleaning and no one is in the lobby or in drive, go for it. You can even wear pins on your apron and leave union material in the back office (towing the line because the managers desk is usually doubles as the break area)

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https://www.nlrb.gov/about-nlrb/rights-we-protect/the-law/employees/your-rights-during-union-organizing

Ask to get that in writing and hold on to it. It's always better to get criminal actions down in writing.

You have a legal right to discuss unions and unionization and any active attempts to supress that type of speech should be reported to the Department of Labour or similar proper authorities

If you can find that communicated in writing, preserve it! It can get them in trouble with the NLRB. Consider sharing it with partners

Already been said
But again
Get that in writing or screenshots or recording and hold onto it.
Discussion of wages and labor organizing are legally protected rights and saying otherwise is illegal.