How to know if a wireless router is able to set local wireless file sharing

How to know if a wireless router is able to set local wireless file sharing

I want to buy a wireless ADSL modem router which also has the ability to
set a wireless network between computers in office. Generally How could I
know if it supports that?
Is it possible to use any wireless modem router for this purpose? Or some
of them just have wireless to share the internet not to connect commuters
to each other? And if it's this way how to find out the modem router is of
each kind? One last thing, Does Any wireless modem router with the ability
of LAN file sharing could share files directly via WIFI too?
Thanks