This is all about routing and straight forward.
- The Router/Bridge has to have the Ubuntu Wlan0 as a default gateway
- Your friends machines need the Ubuntu Wlan0 as a default gateway and get it possibly from the router/bridge (as I don't know the hardware capabilities, you might want to install openwrt on that thing if possible).
- The Ubuntu machine has to be enabled to route the data from nic to nic and has to have the Internet Wifi Router as the default gateway
As I don't know the hw your are using I can only help you with part number three.
Here is a description of how to unlock ip forwarding.
Here ist a description on setting the routes on Ubuntu machines
I have looked at the router and I'm quite sure that your setting can't be achieved with this hardware.
The bridge you mentioned does only "glue" together the LAN with the WLAN. You can't use the ADSL-Port, pretty sure about that.
I found NO evidence that this device is capable of adding additional routes. So I guess that you can't tell this device that the default route is not going through the adsl port but that it should choose a lan port for that.
This could have worked if you would have another compatible device working with openwrt, as it hast so many more options. But unfortunately your devices is not listed there.
Besides from that
To be honest, although it will be possible to achieve your goal, you might not be satfisfied with the performance once you get there.
There is WAY TOO MUCH WLAN in there, making your routes very unstable and therefore slow
If one of your friends decides to stream or bit-torrent in your network it could make the bandwidth so small for veryone else, that it won't be fun to surf anymore. Maybe even collapsing the network.
If it's locationwise possible try to change from wifi to cable whenever possible.
So a much better set up would be (depending on the location of course).
friends --> WLAN ACCESS POINT --> connected to LAN CABLE --> UBUNTU router --> connected to LAN CABLE --> into LAN/WLAN router with ADSL or similar internet connector.
This would be the best for all people involved. If this is not possible another solution would be
friends --> WLAN ACCESS POINT connect to --> WLAN ACCESS POINT Ubuntu --> onnected to LAN CABLE --> into LAN/WLAN router with ADSL or similar internet connector.