I assume they had an informant and/or surveillance
on these guys, but generally "what to do about big groups of people who are, at a minimum, being deliberately intimidating" is not quite solved! At least when they're white.
Authorities have said they arrested 31 members of the white supremacist group Patriot Front near an Idaho Pride event after they were found packed into the back of a U-Haul truck with riot gear.
Seems like they didn't actually have guns, but in places where guns are extremely legal, it's not quite clear what difference that would even make. If guns are extremely legal, 50 guys walking around with guns is extremely legal, too, absent other evidence. There is no good way of dealing with this.