the glitch is caused by using the Retry Checkpoint feature during Chapter 2's Polite Society, Valentine Style mission.
If a checkpoint is reloaded after failing the mission, the four aforementioned characters "will only appear in camp when they have missions for you to complete." For those who've yet to attempt Polite Society, Valentine Style, here's the official workaround from Rockstar:
In the interim, in order to experience all vignettes and conversations, players must complete the 'Polite Society, Valentine Style' mission without using Retry Checkpoint. If the player fails any portion of the mission they should abandon and restart mission, or load a save before the mission failed.
While players will miss out on certain side conversations, Rockstar says the glitch has no impact on actual mission progress and the companions should reappear at the beginning of Chapter 4.