Strider, the leader of a Monolith squad, recently regained their freedom from the Monolith and wants to seek shelter for him and his squad, before they are discovered and attacked by other stalkers due to them wearing Monolith uniforms.
You can find Strider just north of the container warehouses and near the bridge going to the AA complex in Yanov. They are off the road and can be found easily on radar.
You can recruit Strider and his squad into either Freedom or Duty by talking to Loki and Shulga respectively. However one will need to earn the trust of one of the leaders for them to send recruiters.
One only needs to have a friendly reputation with one of the factions to earn the leaders trust, which is essentially one side mission that allows the player to choose which faction to side to or earning the Diplomat achievement.
Strider and his squad taking the Duty oath.
Strider and his squad being debriefed about Freedom's ways and ideals.