We've also encountered merge conflicts on our project. It's daunting to fix merge conflicts line by line especially if most team members you have do not come from a developer background, so I suggested we use "composite" project option. When you use composite, make sure that on loading the project at SoapUI, you select the main folder, and not the XML file. Otherwise, the one you are loading will show "Composite Project: False" when you check the Project Properties. As far as I know, when you save a composite project, different folders will be created per test suite so we were able to avoid merge conflicts since one person works on one test suite while another works on a different suite or resource so even if they work on the same time, conflicts were avoided (we are working on separate folders and files now, instead of one whole XML which generated the conflict before).
Another option I've been told is to use gitignore so that whatever changes others make to one file will be disregarded by my changes but I haven't tried that one yet. I'm also still new with SoapUI and Git :-)
