1. What does it mean?
유입 ordinarily describes an inflow. In community talk it can refer to incoming participants collectively or label someone who has only recently discovered the group or activity.
Also written or said as yuip
2. The nuance
It is not necessarily an insult. A welcoming guide may address 유입, while a dismissive comment can use the same word to question someone's knowledge; tone makes the difference.
3. Examples
“새 영상 덕분에 유입이 늘어서 안내 글을 고쳤어.”
The new video brought in newcomers, so we updated the guide.
“저는 이번 공연 보고 들어온 유입이라 천천히 배우는 중이에요.”
I joined after seeing this concert, so I'm gradually learning how things work.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.