I don't know how good of an idea it is to gulp down a bowl or two of soup right before tripping on mushrooms, but if nausea is not a concern of yours, I'd just follow the volumetric dosing rules.
Blend the mushrooms so they dissolve completely and are more evenly spread throughout the soup. If you use 50 grams of mushrooms for, let's say, 3 liters of total soup weight, and we assume that each bowl your friends are going to consume is comprised of roughly 500ml of liquid (or 2 standard cups), that means you can do the following calculations:
50 grams of mushrooms / 3000ml of soup = 0.0167 grams per ml.
Each serving = 500ml x 0.0167g == 8.35 grams of mushroom per serving
You can replace the 3000ml figure with the amount of soup you intend on making, as well as 500ml with the actual size of servings you intend to serve. Also, make sure you stir the soup
very well before serving to make sure the mushroom pulp is as equally distributed as possible.
Lastly, and this normally goes without saying, but make sure to account for the strain and its potency when thinking about the dosages. 8 grams of liberty caps is a slight cognitive tickle compared to, say, 8 grams of pan cyans which might make you blackout.
Cheers!