I use lentils a lot
Red ones: I used about 3 small handfuls for this latest batch of soup and that will do 5-6 portions. If you want it more of a lentil & veg soup and less of a veg & lentil then probably 1 small handful per portion.
Rinse them in a sieve first, then I bung them in once the veg are all in sweating. If you don't fry the veg first just add them all at the same time. Red lentils take about 20 mins but it varies; they will cook down into a thickener, you won't see individual lentils really, but they add flavour and texture. More importantly they add protein so it makes the soup more of a meal, it will be more filling.
Green lentils: Rinse first, then put in a little saucepan with cold water to cover plus about an inch. Bring to the boil, turn down to simmer; they will take about 25 mins but it varies; they will still be individual lentils, take one out and try it, it should be softish but will still have texture. Add them and their water to the soup once the soup's boiling.
You can also use these to make minced meat stretch - add a handful to make one extra portion