Is there a way to share a wi-fi connection over wi-fi?
In order for a Mac to create a local Wi-Fi network, it needs to have a wired Internet source. So what you need to do is:


Connect an Ethernet cable from the router (the wired source) to a Mac.
On that Mac, choose System Preferences > Sharing.
Select Internet Sharing.
Choose Ethernet from the “Share your connection from” pop-up menu.
Select AirPort in the “To computers using” list, then click AirPort Options and give your network a name and password.
Connect the other computers to the new Wi-Fi network you just created.


If needed, there are more details on this Apple help page: Sharing your Internet connection.