Through my own experience, you can request a guide when you need or desire to, during my strongest meditation, I was able to do this, now im guided by his wisdom when it is needed (and manage to be a fool in my spare time
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I see it this way, a guide is not assigned as such, there are beings that may watch over you from the begginning but I think of them as more over seers. possibly watching over groups of connected spirits, but like I say thats another matter.
If you can see my logic, you should be able to call for a guide, I requested mine personaly by simply desiring his help and preasence after he died, but not out of sorrow, but happyness and love, also curiosity
There are loads of things you can ask of your guide, think of him as a tour guide and enjoy the sights, or seek knowlege you thaught unatainable, you get the jist.
There may be a catch or two depending on your guide, or you may feal obligated to return the kindness by honoring them outside of your meetings, I spread the wisdom of my guide and somtimes rant and ramble on about everything and nothing.
Hope this helps.