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This seems like just another Trump tweet but I don't think people understand what weight this could theoretically carry. I'm inserting no opinions or beliefs into this information, just stating what research brought me to. I AM NOT A LAWYER, JUST HAVE A LOT OF FREE TIME! (Thread) https://twitter.com/realDonaldTrump/status/1267129644228247552
Firstly, this is the list of terrorist organizations currently designated by the Secretary of State as part of the INA. https://www.state.gov/foreign-terrorist-organizations/ There are no domestic terrorist organizations designated by the Secretary of State to my understanding.
Note: The antifa has no clear "base" or location you can pin it to in order to classify it as domestic or foreign, but it would likely be the closest to a domestic terror group to enter this list. Regardless, I'm treating this as though they would receive an FTO designation.
Secondly, it's important to understand that the antifa is amorphous. There is no leadership or centralization, and more importantly, no system of membership or clear way to distinguish involvement. It's more or less just a system of ideas and a decentralized movement.
Two main laws exist to fight Foreign Terrorist Organizations (FTOs). The Immigration and Nationality Act (INA as previously mentioned) & Executive Order 13224 (EO 13224). Theoretically, you can be categorized as an FTO by one & not the other, but we'll say both apply.
According to this State Department fact sheet, here are the consequences of each designation. https://www.state.gov/terrorism-designations-faqs/
As you can see, being designated as an FTO or a member of one can bring up major consequences. This includes seizure of assets, travel restrictions, and deportation. This isn't anything special, it's pretty common for countries to combat terrorism in this way.
Here's where the vagueness of the antifa presents a major problem though. Under INA designation as an FTO, members receive some pretty serious consequences like the one below. But, when there's no clear system of membership for an FTO, then how do you identify involvement?
Long story short, the Secretary of State decides, which usually isn't a big deal. But that leaves a lot of power in the hands of the SoS. Most terror groups have belief systems, hierarchy, and leadership. The antifa only really has the first and, even then, it's not very clear.
Why does that matter? Well someone has to decide on a definition, and depending on who that is, that can cause a serious problem. President Trump has thrown around the name antifa as a blanket term for all protestors in events like this one.
Note: For the record, I honestly don't know who would actually get to decide on a definition of what the antifa is.
Whether he gets to personally decide on the definition or not, he can still certainly muddy the waters. With that in mind, a definition vague or wide enough could encapsulate almost anyone partaking in protests like this.
This brings us back to those consequences of being part of an FTO that I showed before. Being categorized as a member of the antifa could subject protestors including US citizens to a whole world of charges.
The good news: It doesn't seem Trump has any clear authority to unilaterally declare an addition to the FTO list. Further, there is no way that protestors could be mass added to terrorist lists and such an attempt would likely be shot down as unconstitutional.
The bad news: This is without a doubt an attack on the aforementioned constitutional rights and should be taken seriously.
Further disclaimer: Once again I remind you, I'm not a lawyer or a legal professional. I'm just someone fascinated with politics and constitutional law and has access to Google. If I made any errors in this thread I'll correct/delete accordingly. I've attached my sources below.
Frankly, I have no call to action other than do what you think is helpful to bring positive progression. Depending on who you are that can be donating, protesting, posting, or just talking to people. Black lives need to matter for all lives to matter. #BlackLivesMatter 
Sources:
https://www.economist.com/the-economist-explains/2017/07/29/what-is-antifa
https://www.state.gov/terrorist-designations-and-state-sponsors-of-terrorism/
https://www.state.gov/terrorism-designations-faqs/
https://www.state.gov/foreign-terrorist-organizations/
https://www.state.gov/executive-order-13224/
https://www.loc.gov/law/help/peaceful-assembly/us.php
https://www.law.cornell.edu/constitution/articleiii
https://www.economist.com/the-economist-explains/2017/07/29/what-is-antifa
https://www.state.gov/terrorist-designations-and-state-sponsors-of-terrorism/
https://www.state.gov/terrorism-designations-faqs/
https://www.state.gov/foreign-terrorist-organizations/
https://www.state.gov/executive-order-13224/
https://www.loc.gov/law/help/peaceful-assembly/us.php
https://www.law.cornell.edu/constitution/articleiii
