I think the popup only comes around once for users and if you don't accept it, never again, which seems a little unfair. A friend of mine joined BGA and saw it but wasn't ready to pull the trigger. She convinced me to join and mentioned that to me, so when I saw it pop up, I jumped at it. I think it appears only 4-5 days after you join.
I don't understand why there just isn't a free one month trial available whenever people are ready to join. Some people need a bit longer to decide if they want to move to premium (and while I understand a free trial isn't really a full move to premium, it requires payment details, and I get the reticence on doing that less than a week after joining).