I have 5 companies and do payroll out of all 5 can I do payroll out of one and bill the others. Some of the companies only have one employee. Any reason to do separate payrolls.

Are they separate entities?

You can set up a 6th company, that is nothing but a payroll company and that company can contract with the other 5 just to provide payroll services.

You don't want to start paying all employees out of one company, it defeats your purpose of having them separate companies.

Are they separate entities?

Yes, but the have intercompany accounts with one another already

So is it basically all one large company with shared assets and shared liability ?

Are they corporations or private owned and if private owned, owned by same person?

The reason you start separate companies is to separate them for liability reasons.