Please help me welcome Mr. Nomura.
Not completely wrong, but it sounds unnatural a bit.
野村さんを歓迎するのを手伝ってください。= Please join me as a claqueur.
The preparation is already all set. And the speaker is merely asking you to participate and set the mood for the welcome party .
野村さんの歓迎会（かんげいかい）を手伝って下さい。= Please help me prepare for the welcome party.
The speaker is busy to prepare for the welcome party, and needs your help.