When an individual first attempts to join a private group, his or her membership will be listed as "pending." Once an administrator of the group upgrades their membership, they will be able to receive messages, and depending on if the group is moderated or not, send messages.
When someone attempts to join a private group, their membership is initially listed as "pending," signifying that it is not yet active. Upon approval by an administrator, they gain access to message features, which are contingent on the group's moderation status.