The sum of all the other scores. Essentially, you need at least 100 posting, 75 reading, 50 trust, and 7 age score to qualify. But between those 4 scores, you also need a total of 500 to qualify. Realistically, I think the main two blockers are age and trust, not score. By the time you get enough coins to purchase Members+, I think you should have done enough posting and reading to get the effective score you need. So the only two factors you can't really control are trust and age. Trust is purposefully obscured, so we don't know what actions can increase it. So if you somehow lose trust, and get below that 50 threshold, there's not much you can do except ask an admin for help. As for age, it's a hard 7 day limit, and you can't exactly cheat that.