Official SSQRG membership can be obtained through applying for, RGS-IBG Fellowship, or RGS-IBG Associate Fellowship (if you are a postgraduate or are within five years of graduating from your first degree), and, then, select SSQRG on the join research group form per guidance at this link.
Either RGS-IBG or non-RGS-IBG members can sign up for membership of the SSQRG listserv here.