He later ends up revealing to Yoojin that his item never needed an invitation like the others as he was already presumed to go, his item doesn't need to have such frivolous things as being invited as he's allowed to come and go freely with SHJ. This was after Yoojin was so grumpy and petty about the only one not being invited to his party, that he goes to SHJ's garden rooftop and dump a sh*t ton of hot pink dyed yarn on his face, and when I say a sh*t ton I meant ALOT, he ends up using it to knit alot of clothes and accessories for yoojin ( I wish I was making this up, in the novel it's soooo much more domesticated and BL coded)
*Spoiler* P.S, it's heavily implied that Yoojin's favorite color is pink, he mostly ends up wearing it or drawn to the color in the series but never outright confirms it. Also if you remember the magic stone embedded in yoojin with SHJ as the "parent" or base material? The kid, Han-Sung Gyeol who looks extremely like the Seseong guild master ends up having Pink hair and its explained that Yoojin just really wanted him to be born with it subconsciously
GAAAAAAH THE BIRTHDAY SCENE FINALLY HAPPENED, can't wait for the aftermath of the party where SHJ straight up says he's endeared by him or smth. PLEASE MANWHA ARTIST, DON'T REMOVE THAT SCENE