Here are the number of voters registered in various unqualified parties in Massachusetts, just prior to the November 2018 election. The Massachusetts Secretary of State does not put these totals on his web page, and it took a while to obtain the data.
The Green Party became ballot-qualified on election day, November 6, 2018, but it wasn’t ballot-qualified when this data was compiled. This tally is as of October 17, 2018:
Green 4,970
Socialist 1,037
Constitution 265
Reform 128
Working Families 55
Prohibition 16
The August 15 totals for the same parties were:
Green 4,932
Socialist 808
Constitution 244
Reform 119
Working Families 12
Prohibition 18