Typically an employee must be allowed to participate in the 401k after obtaining 21 years of age and one year of service. One year of service is defined as 1,000 hours in any calendar year, plan year or rolling 12-month period of time.
Plans can be modified to have less restrictive eligibility requirements.
401k plans cost around $1,000 annually for the plan administration from a TPA (third party administrator) and there are asset management fees of 0.5% (index funds) to 1.5% (managed mutual funds) as well.
So you will have two vendors with a traditional or company-sponsored 401k plan. You will have a self-employed 401k plan administrator and you will have a custodian / asset manager. These are oftentimes handled together, but have several clients who custody the assets with Vanguard, for example, yet maintain the plan administration, like filing Form 5500, themselves. Seems like a lot of work.
Be very leery of ADP, Paychex and Wells Fargo. Those are the top 401k plans that are lost to competitors who offer better customer service, better choices and overall better plans for you and your employees. Wells Fargo is notorious for offering “free” 401k plans, and once you are committed, you discover that the plan is very limiting and underperforms. Free? Really? When is the last time you received something for free that was worth keeping? You don’t work for free, so be careful of those who claim to.
Taxpayer's Comprehensive Guide to LLCs and S Corps : 2019 Edition