There are several ways to accomplish this. With a limited budget and no dedicated IT department, I recommend using a NAS (Network Attached Storage) drive. You can provide everyone their own backup storage on the NAS and provide security via passwords. You can also create a public folder which you can provide access to the occasional visitor. To prevent the visitor from gaining access to private material you will need to kill file sharing on every computer on the lan.

A NAS used to be expensive. You can now purchase a NAS for less than $150. If you have an extra hard drive laying around, you can buy an inexpensive NAS capable enclosure and use your own drive.