Prejoin is a general term for the early opening of a TCG. Many owners will allow members to register before the website is completely finished, and players who prejoin are typically given extra perks or a larger starter pack upon joining. Prejoiners are also almost always marked on the member list.
Background[]
It's not known which TCG first implemented this idea or even came up with the name. Regardless, having some form of a prejoin period has become a standard that almost every TCG now follows.
Time Period[]
How long the prejoin period lasts varies among TCGs. Some owners sometimes set dates for when prejoin will end, others will cut it off after a certain number of members join. Humorously, some TCGs don't leave the prejoin period and run normally for months without ever having a grand opening.
Cards & Decks[]
Some TCGs go the extra mile to create a prejoin event card or deck. At times even the prejoin decks are created with images donated by the prejoiners.
Badges[]
Some TCGs celebrate their prejoin period by offering custom prejoin badges to each of their prejoiners.