If you have a staff, but you do not want to deploy a company-sponsored 401k plan, you can still maintain an owners only self-employed 401k plan by excluding employees.
If one of these conditions is true, then you can maintain your solo 401k plan.
Don’t go out and try to make your admin an independent contractor. In most states, to maintain independent contractor status, the person must hold themselves out to the public as a contractor in that trade or profession. We don’t see too many admins running around with business cards and websites advertising admin services.
There is wiggle room. First, there are PEO (professional employee organizations) which allows you to hire the leased employees, but the PEO runs payroll and handles all the human resource functions. You now have an arms-length relationship with a person, and can truly call them a contractor. There is some mounting pressure on PEOs since they help small business owners avoid or reduce a lot of things such as unemployment compensation insurance, workers compensation premiums, fringe benefits, health care, and retirement plans.
You can also have everyone in your office be licensed. For example, insurance agents, financial advisors or real estate agents might work together as a team, but with revenue sharing capabilities an independent contractor status can be maintained while effecting certain “control.” This gets tricky and is a narrow example.
Taxpayer's Comprehensive Guide to LLCs and S Corps : 2019 Edition