Setting up bridging on OpenWRT is described in detail here:

http://wiki.openwrt.org/doc/uci/network

But the basic idea is to edit the file /etc/config/network, create an entry for your desired bridge (probably something like this, slightly different if you want to use DHCP to assign the address):

config 'interface' 'br0'

option 'type' 'bridge'

option 'ifname' 'eth0 wlan0'

option 'proto' 'static'

option 'ipaddr' '192.168.1.100'

option 'netmask' '255.255.255.0'

option 'dns' '192.168.1.1'

option 'gateway' '192.168.1.1'

I'm assuming you have an entry for the wireless interface (and assuming that it is called wlan0) in /etc/config/wireless. Modify as appropriate.

Then restart by issuing the command

/etc/init.d/network reload

This may not be enough to do what you want - do you need multicast routing etc. as well or just want to multicast to a group within your local network?