“Wonder how much damage they will do”
It depends on a number of factors, including how long they are there, what the other committee members do and how vocal or violent their supporters are. It is entirely possible that their communist supporters will threaten other committee members and, as a result, more ridiculous, economy destroying legislation will make it to the floor. Then, the entire House, Senate and the President must agree. So, in the short term, probably not a lot of damage. In the long term, it depends on how the next election redistributes our governing philosophy.
America is at most two elections away, at any given time, from being a Communist country. It took Russia about eighty years to begin undoing that mistake. But, look at them now. Their lot has hardly improved. It is much better that every generation of Americans fight hard to keep what we have, as it is easier and less bloody than losing it and trying to get it back. We have lost the first two battles, the education system and gay marriage. Gay marriage was never about gay marriage. If it was we would not have all the lawsuits against bakers and companies promoting wokeness. It was always about total nihilism.
The CIA fought international communism for decades, only to shepherd it into the U.S. Just for power?