He's still a newcomer, so he isn't used to the work yet.
We made a manual that's easy to understand even for new employees.
古参 refers to someone who has been in a group a long time, while 新参 refers to a newcomer who has just joined.
Compound of 新 (new) and 参 (joining, participating). The exact historical derivation is uncertain, but the kanji combination directly suggests 'newly joined'.