Antifa is most definitely an organization. The claim that it is an activity and not an organization is just low-tier gaslighting. Here's hard evidence: https://en.wikipedia.org/wiki/Rose_City_Antifa
As noted, people may join together for anti-fascist activity, and any organization may participate in such activity. That does not imply an "Antifa" organization any more than the activity of fascists implies a Fascist organization.
There are many fascist and fascist-leaning groups and individuals, and many opposing them. Anyone may oppose them, without need for permission. Organizing with local others can produce greater effect. That also does not require permission.
Any existing organization may choose to engage in anti-fascist activity, possibly in coördination with other groups. That also does not imply an "Antifa" organization.
Opposition to fascism is everyone's responsibility. How that opposition may best be effected varies with circumstance. Rooting out fascism in police departments is an obviously important part of anti-fascist activity.
