At some point, you will be on an outing and someone doesn't have a lock, or didn't bring it, or left it behind, or lost the key, or it broke. What do you do? First off, you should have brought a lock.
Leaving someone to guard the bikes will be very helpful. It is possible that a thief will overpower the guard there are multiple thieves who decide to try to steal multiple bikes.
Group all the bikes tight together so they are easy to keep an eye on and so its a little harder to get to some of them.
You may be able to use an extra cable to lock multiple bikes together.
If you have a lock, but nothing to lock it to, lock bikes together so they can't be easily ridden or carried off. Ideally lock a wheel to the frame so that it can be ridden off.
Anything you do to slow a thief may give you a little extra time to take action and stop a theft.
Helmet Straps
Use Helmet start to secure bike to anchor or at least prevent a wheel from moving
Paracord
Use Paracord and a Scout knot to secure your bike
Strap and Carabiner
Using a strap and carabiner is a common way to deter thieves from snatching your backpack
A strap and carabiner can be stored in your seat pouch and used to secure a bike to a post or tie gear to your bike
Zip Ties
They make locks that are basically fancy zip ties
These will slow a thief or at least stop one that doesn't have any tools
Actual zip ties can also be used; you'll just need something to remove them with
Gears
Put bike in highest gear before stopping
It will be a pain to get going again...but that's the point
Chain
Take chain off sprockets
The thief will have to put the chain back on before riding your bike
This is a pain because there is a good chance you will get your fingers greasy
Hills
Park your bike facing uphill
If you are on a steep hill, a thief will have to turn the bike around to get going
Disable Wheel
If you have a quick release, remove it or remove the wheel and take it with you
The theif will need to carry your bike away
Seat Post
Removing a seat post makes the bike less desirable to take
Flipping the Bike Upside Down
Flipping the Bike Upside Down add one more step for the thief and draws some attention
This works better if bikes are crowded together
Uglifying a bike isn't for everyone, but making a bike look really repulsive will help deter a theft
Some bikes are just used for joyrides and turn up later. Take a few steps to make it easier to recover.
Record the Serial Number
Take pictures of your bike
Place Airtag tracker on bike
Register your bike