When using the Entralink feature to visit another player's Black City, the player can invite people who have left their Black City to come to White Forest to increase occupancy; this can enable new items and wild Pokémon to be found.
White Forest has a maximum population of 25, although only 10 residents can be present at any one time.
Each NPC [Non-Playable Character] has a set number of points, which will change depending on certain functions that can be performed by the player. If an NPC's points value reaches 0, then they will leave White Forest. The factors involved in this are as follows:
New NPCs can be brought into White Forest by inviting them from Black City via the Entralink.
NPCs that have left will be visible to other players in Entralink. If this area becomes full, no more will leave, even if their score reaches zero.
Source: Bulbapedia/ White Forest
Happy NPC Hunting!!