In your career, you have probably worked very closely with certain companies, and you want to connect with your friends there. You could connect with them easily if you added that company to your profile, but since you were never actually an employee, that company really does not belong in in your LinkedIn profile.
Ready for the hack? Just momentarily add the company to your profile, search for your friends at that company and invite them to connect. Then delete the company from your profile before anyone sees it.
Abusing this hack will probably get you kicked off LinkedIn, as it should. Only invite people who would welcome an invitation from you.
Still, leave as many companies as possible in your profile, so your former colleagues can find you in the future.