My company is very small, about 10 people. We have our 401k plan with Guideline and they've been great so far! The cost is a one time $500 fee, plus $8/person/month. Their website is very user friendly, customer service is stellar and all their funds are Vanguard with no extra expenses for employees other than the usual Vanguard expense ratio. They're even willing to show you a demo to help you decide whether you'd like to use their services. The only downside is that the only accept payroll contributions.
Hope this helps!