There are a number of good answers that deal with how imprints might be made while the people are living, that can then be recycled into guides when the person is dead. Several generations down the road this covers a lot of immediately known ancestors. You mentioned in one of the comments you wanted this to work on ancestors who had died before the alien intervention occurred - since that's trickier, I'll focus on that.
So there are a lot of theories on how ghostly imprints (or residual haunting) might be a kind of natural recording - some minerals, geographic formations, and locations under certain circumstances might 'record' whatever is happening, and then under the right stimulus, probably requiring some access to energy, the area can 'play back' whatever was recorded. Often, the circumstances that lead to such recordings are tied to strong emotion. Gems and crystals are also sometimes said to be able to store this kind of emotional or psychic imprint. Don't worry, it's already in the question that you wanted something more interactive than these kinds of traces, but I thought this could be paired with your other requirements to make a solution.
So, if your people before the alien intervention had a couple of beliefs and practices, this can help make the whole thing work. They need a strong tradition of venerating ancestors, including praying to these ancestors for guidance, a sense of duty to their descendants, and the idea that lasting, living memory is somehow useful to spirits. They also need some idea or superstition of the stone tape type of residual imprints, which they happen to successfully use - I'm thinking some ritual jewelry, which happens to be made of the right crystals and the right mix of minerals, which is ritually given and worn every day of their lives, and which becomes the focus of their family's memory rituals after their death. So these crystals have some kind of emotional imprint or overlapping recording of the person's most intense emotional moments, as well as a bit of how others in their family remember them.
So, during alien intervention, a computer (or several) get dropped on your people. It might be bulky, or self replicating, or dropped in all differing locations, or have a really long range, or be actually connected to the Pactite, I don't actually know. Programs get generated for each Guided One - and maybe they were supposed to guide them (as some kind of social experiment from the aliens) or maybe they were only supposed to observe. As it happens, it uses some vaguely-similar crystal technology to store information - and predictably, that means it somehow actually picks up and interacts with the recorded imprints from the burial crystals. A program which has some of the memories, will self-identify as the person, and seek other memories that fit and discard those that don't, so it ends up one set per program even if they weren't actually programmed that way. Alternatively, once one program 'wakes up' it may deliberately try to pair the memory recordings with other programs either because it thinks that's how the programs are supposed to work, or because the person it's acting like realizes what's happened, and waking others will help their families. The two sets of stored information collide, and programs "wake up" with memories, emotional memories, of the people in the recordings.
This is starting from imperfect, static imprints and recordings, mind, but once the program self-identifies with the person, they can seek out other information about the person (from records, and living memories of Guided Ones), and develop their personalities based on the impressions they remember. And since they already have beliefs that ancestors are supposed to help guide and teach their descendants, they make the situation fit even if it wasn't what the programs and computer were originally supposed to be doing - which is why I mentioned that the programs might not have been meant to guide, but only observe or even other things (since this is a really tricky thing to intend). Once the programs have emotional memories, they will self-identify with the person and act as the person would, not necessarily as the program should. They might also resist deletion once their guided one is dead, and prefer to work with only their own bloodlines if they can. The society that has them might advance more quickly by having ancestors given time and memory storage to think about and plan new things, to not lose bits of information to death or forgetfulness, to simply have more minds working on any given project, or even by having some access to things stored in the alien computer - with strict limitations on what's actually in the computer, what they can understand, and what they can use.
So, my first thought was the Pactite itself was the source of the computer and programs directly - nanomachines or the like. However, since you mention in a comment that you might not want everyone to be dosed with nanotech, maybe there's another solution. Maybe people in your world have a low-level telepathic ability - obviously a science-compliant one. Low level enough to be subconscious and have people be mostly unaware of it (maybe it comes out in hunches or intuition). Perhaps that is why the people could successfully make the stone tapes for recording in the first place. In this case, the Pactite might boost the native ability long enough form a link or alter their minds to make a permanent conduit through which their Guide can find them. Their native ability might be enough for maintaining the link once it's set up. On the other hand, if ingesting more Pactite just boosts the telepathy, that wouldn't necessarily create a second link - but it might be that no one does ingest it without that intention, so the second guide is actively trying to create the link at that time. Or maybe it's not actually telepathy, but somehow the combination of the Pactite and the rituals let a (wireless) link be built to a main computer, where the programs live.
As a bonus, the period of time in which all this is getting set up (the memories meeting the programs) would seem to be the ancestor spirits adapting to the boost the Pactite gives (and the changing rituals), and figuring out dosage and empowering on both sides, for the guide and guided. The framework for ancestral guidance is already present, and if they already have coming-of-age rituals incorporating the Pactite and guides can be easily done once they have some ideas for what works. And, double bonus, this doesn't have to be something the aliens intended, per se (as it is a strange and difficult thing to do on purpose). The aliens might come back and discover their observation computer has been taken over by programs, which somehow have emotional attachments to their subject and identity disorders, and be very confused.