Assuming you've followed the multiplayer tutorial, the answer to your question can be found here. https://www.scirra.com/tutorials/915/mu ... ame/page-7
If you have your inputs set up correctly, then the host can get the peer inputs using the multiplayer.peerstate expression. Then the host can proceed with actions per peer (creating projectiles, moving) depending on each peer's inputs. You bullet will also need to be set up ahead of time as a synced object.
I'm going to add a bit of advanced info here, as I've gone through this process before, and I'm guessing you'll run into issues regarding lag compensation and input prediction like I did. Basically you'll want your synced, authoritative projectile to be invisible, and have a local representation of your projectile be displayed immediately for the peer which gets it's position corrected over time to the synced projectile. Otherwise the lag time for bullet creation and destruction is pretty horrendous even on a 0 latency connection because of the way interpolation is built in to the plugin.
If you have no idea what I'm talking about, don't worry about it. You'll get there eventually <img src="{SMILIES_PATH}/icon_e_wink.gif" alt=";)" title="Wink">