Jin-woo wants to solo clear the dungeons so, he wants someone who won't go to the dungeons with him. The minimum headcount to form a guild is three so, he only wanted to fill the count.
If an S rank hunter like our best girl joined the guild, he wouldn't be able to solo a dungeon and gain more power.
The reason why jinwoo accept sujin's cousin is because she's just a hunter by name and doesn't care about raiding. unlike fmc she's a real hunter. jinwoo want solo dungeon that's why he rejected fmc.