It's great that you want to refer someone to Prolific! You are very welcome to recommend to a friend that they join our waitlist.
We turned off our participant referral scheme back in 2019. Our aim was, and still is, to offer a high quality participant pool with a great range of diversity. We noticed that referrals were working against this, creating larger groups of the same demographics. In order to encourage balance, we turned off the referral scheme.
This change has been positive for both researchers and participants as we can distribute studies across our pool more evenly. This means more studies for all of our participants!