The wiki article alone is exciting reading, considering how many different pendulum systems were over time, with different uses, different properties of those systems.
When using it as a plot device for determining length, you just limited by the wish how complex the system can be and how much work they should put to make the system and extract results.
According to the wiki article, it was proposed to use the pendulum to establish a unit of length based on pendulum properties, before it was become known that gravity varies slightly from place to place. And it took them some time to measure the variations, so you can imagine how small is the difference.
They may or may not know parameters of the planet and thus by measuring angles of stars and sun and synchronizing over the radio they can determine their latitudes and eliminate the uncertainty involved by rotation of the planet thus precision will be limited by local deviations which is relatively small.
Gulf gravimeter in the article reported being able to measure gravity with the accuracy of (0.3–0.5)×10−7, thus if we assume equal gravity and account things we can account, the accuracy can be length accuracy.
Pendulum was a part of clock design for a long time, and I definitely saw on the YouTube the instruction of a master survivalist how to make a pendulum clock, ClickSpring playlist, Making A Large Wheel Skeleton Clock
Pendulum system does not require precise intervals or time interval synchronization because you can measure not a single time interval, but many time intervals, basically as much as you would like to have, but at least you count oscillations of a minute or two and compare the number of oscillations in the period.
Also, it allows fine tuning of the length with simple means - regulator
NB clock making does not require common measuring system, they can choose arbitrary unit at the initial stage.
If you have pipes you can make atmospheric pressure gauge, for water you need about 10m, for mercury about 0.760m.
Mercury gauges are relatively simple if you have glass and have ores of mercury.
Mercury extraction is just heating the ore in enclosed vessel and condensing it.
Cinnabar is mercury ore (HgS) - has very distinctive brilliant red color. So if you would like them to do some digging.
Measuring pressure is also useful to forecast weather conditions, so they may wish it to have for those goals too.
By averaging pressure values they can establish precise enough units of length.
They may or may not know the circumference of the planet, but they can measure it in initially arbitrary units by repeating the experiment.
You can use the way if you would like them to travel a lot(far enough)
Precision, what is enough
Practically you do not need a super high precision of establishing those units, they should be about right.
If the master survivalist is willing to guide the group A to create their means of survival the units has to be about right but not necessary the same.
The reason for it is that methods of producing and manufacturing things do not depend on the unit of length most of the time. They begin to be more or less important in things where tensile strength is important and safety factor is low (stressed constructions used to maximum of their capabilities)
It might be important to have a master length in each location, and it might be important to have similar lengths to make fewer adjustments later when they synchronize their master length by exchanging physical object.
0.1% accuracy in establishing initial units means 0.1mm difference of a 10cm detail or piece of machinery - in a lot of cases, it's not a bad accuracy, especially for simple machines and devices. And realistically speaking if they start from zero, such accuracy is just great. It is definitely a good accuracy for up to 1900-1920 tech levels and in a lot of cases today.
And if you build engine, and if you have only one factory, it is important to have the master length in the factory, if you produce multiple copies of the engine or wish to have a repair kit for the engine. But that all is about having accurate repeatability, not about the absolute length of the unit. And those master length has started about that way, not exactly informative in terms of how to do things but interesting in terms of piece of history Don Bailey, at A.A. Jonson metrology quarters
Also, I recommend another master survivalist and his playlist for different techniques, prototyping/manufacturing techniques for lab applications from Dan Gelbart
It is possible to make DIY lasers such as Carbon dioxide laser, Ruby laser
Synthetic Ruby interesting article about subj. Another master survivalist talks about methods used to make crystals, Lab-Grown Rubies and Sapphires: Flux Vs. Flame Fusion
So, starting from relatively imperfect measuring length they can develop means to establish length unit in wavelength units without exchanging physical implementation of the length. And this is another reason to not seek for exactly precise units from the start. Anyway, people need time to learn skills and knowledge before they can implement and learn technologies which require precise measurements, and when they master skills they can create equipment which allows them to establish length in wavelength units.
It will require optics, lasers, mirrors but not so much of electronics as an example(zero electronics). It might be not that precise as we might do today(or better to say convenient to measure and replicate the length and account for temperature and such), but precision will be more than enough for most of the shops and factories of today.