How to allow access between remote IPSec endpoints?

Greetings,

I have a FortiGate at the main office, subnet 192.168.1.0

There are several hardware-based IPSec VPNs for remote locations:

Remote office 2, subnet 192.168.2.0

Remote office 3, subnet 192.168.3.0

Remote office 4, subnet 192.168.4.0

and so on.

The main office can reach each of the remote offices.

How can I allow each of the remote offices to reach each other?

Thank you,

Steve