401k Plan

An employee was excluded from plan.

I know they must receive a nonelective employer contribution, but how do you determine the amount?

It is also a 3% safe harbor plan, so they get that piece as well.

Start with the average NHCE ADP for the year in which the deferral opportunity was missed plus the match if one was made.  A copy of Rev Proc 2019-19 is attached.

Rev Proc 2019-19 Expanded SCP.pdf

Rev. Proc. 2019 explains everything you have to do, including providing a notice to affected participants.  Also, there are 3 classes which require different actions.  Within 3 Months, More Than 3 Month, and After the Close of the Following Year.  Since JackS was kind enough to provide you with a copy, I leave out additional details.