You get a bonus for owning multiple Regulars in one company. For every 1% of the company you own, you will get a 10% bump in salary for those Regulars. For example, if company A has 100 employees, and you own 4 Regulars that work for company A, they will each receive a 40% salary boost.
Owning multiple Regulars that are employed at the same company will boost their salaries. For every 1% of a company you own, each Regular you own that works for that company will receive a 10% boost to their salary. Ownership of a company is calculated by dividing the number of Jobs you own in a particular company by the total number of Jobs that company has, rounded down to nearest full percentage. A teamwork bonus calculator is embedded here.