You shouldn't have multiple people working on cap files unless it's for prototyping only, because you will have to do all the work over again in the master cap file since construct can't copy paste .cap data. If you want to collab, make one person make the art, one make the sounds, one programs in construct etc. WARNING: DON'T ALL WORK ON DIFFERENT CAPS THEN EXPECT TO IMPORT EVERYONES WORK.
As people have said, the only way to make one person design levels without working on the cap is to make a custom level editor once all the art and code are done.
edit: You can make the level editors in construct, it's a powerful tool.
Also, with 98.9 and lower versions, all images are loaded at runtime. With newer builds (however they are buggy atm) one can choose to load images into vram per layout, all at once, or manually (< I think..)