Sure, it is a live, one-way announcement to a group of phones.
Directly from system help:
Group Page allows you to make an announcement to a group of people through the speaker of their telephones. A tone is heard at both the paging telephone and the paged telephone before the Group Page is established. During a Group Page, the paging set displays the number of the group being paged; the paged sets do not display any paging information.
Open the on-line help and navigate to System Features - Features D to G - Group Page
You can also add in a 5485 IP Paging adapter, so yes you can page phones and overhead simultaneously.