PAN
wireless
Usually wireless adapters can be connected to computers not to modems.
It depends on model. Some modems come with a wireless router. If yours has wireless interface, you will able to connect your computers wirelessly. Usually what you have is a modem with one LAN port. If you want to connect your 3 computers to the same modem you need to get a wireless router. It will allow you to connect your computers using wireless or cable. Switch in such case might not work because of internet provider policy.
Many Wireless routers and switches have few inbuilt switching ports capable of connecting Wired computers to Wireless Computers on the same network to connect wired Computers also alongwith. (Not necessarily every every wireless router will be having switching ports)
You can connect them with an ethernet cable, or by WiFi if they have wireless cards.
It would depend on how far away the computers would be from your wireless network. You can connect multiple printers to a wireless printer, I am sure if it is a small business you can connect all.
Get 2 USB or Internal wireless receivers and connect both computers wireless to the wireless router
You will need a router, wireless adapter, to have a network connecting computers
Unlikely, but i do know that there is a wireless receiver for computers to pick up wireless controllers and headsets.
for connecting 4 computers you would need a "hub"(en.wikipedia.org/wiki/Network_hub), and a cat5 cable to connect each computer to the "hub". each computer would also need a "network card"(en.wikipedia.org/wiki/Network_card), alot of modern computers come with a network card integrated so you may not need to purchase it separately.
You need to connect your modem to a ROUTER, and then connect both desktops to the router, instead of connecting one desktop directly to the modem. You can connect the 2 desktops to the router either with standard CAT5 network wiring (recommended), or wirelessly if you have a wireless router and suitable wireless hardware attached to the desktops.
It is likely that your company wireless is secure. You will need to contact your administrator for assistance.