I've never used it in soap, but when I buy some, I chop the whole bunch and freeze it on a jelly roll pan. Once frozen, I pour the loose pieces into a Ziploc bag and seal it for future use. It works well in cooked dishes.
I haven't used celery, but I'd treat it like any other fruit/veggie: blend it up and strain it, freeze it, and consider it part of my water. I like to prevent gel on fruit/veggie soaps due to extra sugar, but that might not be necessary with celery since i don;t think it has very much sugar in it.
Celery is a wonderful source of sodium. I've never used it in soap, but I always include some in my daily smoothie since I grow it in my backyard. If you soap it, just make sure to blend it and strain it beforehand as Snappyllama said. Let us know how it goes!
A jelly roll pan? I will have to look into what that is.
I have not used celery, but I do recall reading about it sometime in the past. Celery has a very high water content, but the fibrous strings would be undesirable in soap, so grinding it up to a very fine liquid, then straining out the fiber would be best. Then use it in place of water in your lye solution.
Here is one link about a celery soap. Although, I am pretty sure the green tint one may get from celery fades over time, as most botanicals do, so I would add another colorant to keep the green color if desired.
I also saw it used in a body scrub, if you want to find additional uses for celery. Here is a link to a chapter in an ebook with some recipes using celery. I have never tried it myself. I do make body scrubs sometimes, but don't use them very often.
Me, I prefer to use it making SOUP. I make soup from scratch all the time (not a big fan of canned soups) and celery is an important ingredient in soup, for me anyway.
Yup, it goes into one my my soaps. Does it add anything great probably not, but I use it with several other, mostly for label appeal. I never advertise it as doing anything other than cleaning. I put it a blender or bullet puree and strain out the strings. If someone finds the strings in soap they may think it is hair. Whether it adds any hardness I would doubt it even though celery does contain sodium, but my bar is a brine bar so I have no way of telling if it makes a difference with hardness.
I dry celery leaves all the time and make celery salt with it. I use it all the time to savor soup, eggs, anything. When I make my salty sailor salt bar I use the celery salt in the soap so it has pretty green flakes.