They've been running regular anti-austerity, anti TTIP type posts on facebook, pushing left wing policies etc. so it stands to reason that those who're joining are doing so in response to this line of campaigning rather than due to their record in Brighton.
Also, anecdotally, all those I know who've publicly supported them in the council elections etc have done it from a left perspective. The likes of Another Angry Voice has been pushing the green party quite heavily from a left perspective, and is fairly influential with the facebook generation etc.
And the reports from scotland that Ferrel mentioned.
But I'm aware that my experience isn't necessarily indicative of the overall situation, and it's entirely possible they're tailoring facebook adverts to different audiences or something, so wanted to verify this.