Hi Kevin,
You simply need to buy a Wireless Access Point and connect it to your existing network or router. This is assuming that you have an internet connection already established. These access points tend to be very inexpensive and you can pick one up on amazon for about 30 - 50 Bucks.
Given the size of the network and amount of devices that you want to connect, the Wireless router that you get from yor ISP will most likely stand up to the task. If you later expand the network, you may want to consider then buying a dedicated WAP and possibly another switch for more devices.
Once you have enabled a Wi-Fi Hotspot in your office, you will need to connect all of the devices to the network. At that point, they should all behave as if they were wired together and you can start sharing!