Massachusetts Secretary of State never releases a tally of how many registered voters there are in each party, except just prior to a statewide partisan primary, or a statewide general election. The first tally since 2016 is now out. It is as of August 15, 2018. Democratic 33.22%; Republican 10.37%; Libertarian .32%; Green .11%; members of other unqualified parties .91%; independent and miscellaneous 55.08%.
The percentages just before the 2016 general election were: Democratic 34.18%; Republican 10.68%; Libertarian .20%; Green .14%.
The Secretary of State has put the data for the three qualified parties (Democratic, Republican, and Libertarian) on his web page. However his web page does not show the totals for the unqualified parties, even though the data exists.
The number of registered voters in smaller unqualified parties currently is: Socialist 808; Constitution 244; Reform 119; Prohibition 18; Working Families 12. Most of the registrants in the unqualified parties are in the United Independent Party, which is defunct but which was on the ballot 2014-2016.