It depends what they do at the meeting. If someone sends in a spy to record the event, and catches people on recording making realistic concrete plans to commit crimes, then it’s definitely illegal.

The banner seems to be just barely within the range of protected free speech. “The Fuhrer” obviously refers to Hitler, since the event is on April 20. But it seems legal in the US. If they were challenged, they would claim it’s an ad for a peaceful meeting, which is protected.

But if they actually said “Hitler”, or “Nazi”, or put up swastikas or Nazi flags, then they would be within the realm of possibly-not-free-speech, since those can be interpreted as calls to violence. However, it’s ambiguous. There are multiple ways to interpret a swastika, and fascists in the US are really good at feigning innocence.

It’s definitely illegal once they start saying things like “let’s kill some [insert slur here]”