Yes, they needed to be RAs or TAs (including hourly appointments or fellowship appointments accompanied by a partial RA/TA appointment).

https://mitgsu.org/updates/we-have-an-election-date-vote-yes...

Note that for instance if you're there with a fellowship and aren't getting a partial RA/TA assignment as well then you're not eligible. I know that some masters students pay for their tuition, so they might not be eligible either since they're not getting a stipend.