David Beito in his excellent book, From Mutual Aid to the Welfare State, tells us that it is very difficult to estimate just how many people were affiliated with franternal and mutual aid societies back in their heyday – especially since large segments of the black and immigrant population would have been undercounted. Nonetheless, by 1910 he estimates that the combined membership of all types of fraternal societies was at least 13 million (p. 14).
How large is this? In 1910, the entire US population was 92.4 million. So 14.1% of the total population was a member of a mutual aid or fraternal society. That would correspond to about 43.7 million people today. Of course, membership in those societies conferred benefits to the families of the members, so the number of people covered by mutual aid benefits and relief is easily over twice this many. Beito himself suggests a conservative estimate that one-third of all males over the age of 19 were members in 1910, given the marriage rates and larger family sizes, I’d venture a guess that over 40% of Americans have some form of coverage from a mutual aid or fraternal society 100 years ago.