To view Illinois firearms laws go to the Illinois State Police website www.isp.state.il.us Go to FOID/firearm information click on Unlawful use of weapons 720 ILCS 5/24 go down to Section 24-1.6 and read "except when on his own land or in his own abode, legal dwelling, or fixed place of business" If your community has some local firearms ordinances they might have restrictions on carry in your business. I don't know if any do. You could call your city clerk, if you live in a city, or the city attorney could tell you. Also the ISP website has a list of cities that have ordinances listed there. I believe there are some in the Chicago area that don't have theirs posted, though they are required to.
To out of state folks, you may open carry or conceal carry on a person's property in Illinois with their permission. Local ordinances would apply however.