import pandas as pd

df = pd.DataFrame.from_csv(r"append_test.txt", sep='\t')

frames = []
for meeting in range(len(df.T)):  # iterate over recorded attendances
    attendees = df[df.T.iloc[meeting] == 1].index  # get attendees for specific meeting
    frames.append(pd.DataFrame(index=attendees, columns=attendees).fillna(1))  # create frame from attendees

# sum up jointly attended meetings
pd.concat(frames).fillna(0).groupby(level=0).sum().to_csv(r"jointly_attended.txt", sep='\t')