There are many ways to contribute.
Currently there isn't any CI system and with the increasing number of apps there is always the need to test on different or new hardware. Pick some hardware. Try installing the system onto it.
Testing of the mesh system in various environments. What's the maximum range for a given wifi adapter? What type of cantennas or reflectors work best on an ultra-low budget? Which wifi adapters have free software drivers? What are the simplest antenna designs which are quickest to make? Perhaps antenna advice or example deployment descriptions could be part of the documentation.
The project doesn't have much of this. There are some desktop backgrounds within the img/backgrounds subdirectory which could be improved. Cute mascots and things like that can also help to attract interest. The mesh variant of the system has desktop icons which could also be better.
Looking for any obvious security mistakes, doing pentesting on an installed test system and reporting the results would be useful. There are already many STIG tests in the tests subdirectory, but having more wouldn't hurt.
Ensuring that the internet doesn't become far less neutral than it already is. Encouraging ISPs not to have policies which ban people from running servers. Promoting and raising awareness that self-hosting is a thing which is actually useful. All of these activities are incredibly important to allow self-hosting to remain a viable possibility. ISPs are the bottleneck, and if they implement bad government mandated policies then it may become no longer practical or legal to run your own internet systems on your own hardware in your own home.
Typically apps are pegged to a known good commit. One useful thing is to try recent commits and see if the app installs successfully. Do any new packages need to be installed, or old ones removed? See the developer's guide for how to add new apps to the system.
It's all just bash scripts and the more eyeballs on it the more likely that mistakes will be found and fixed.
Just blogging about the project can help to inform people that decentralised systems exist and that they don't need to be trapped in the cloud services of $bigcorp. Even if you find some aspect of the project which sucks badly, blogging about it is one way to provide feedback which could lead to future improvements being made.