A new employer was added to a mutiple employer plan. This is the employer's first plan year. Currently the other employers in the plan are filing a 5500 R. Since this is the employer's first plan year should a 5500 C be filed for the new employer?

Instead of wrestling with that question, I would file a 5500 C for all participating employers for the year in question. The IRS instructions say that a plan sponsor has the option of changing the filing cycle for its plan by filing a Form 5500-C before the form is otherwise due. That would put everyone on the same C/R cycle and prevent confusion.