You're going to hate playing Rathma in groups because you're going to have Blood Lance builds that will skip all trash and leave you behind. So, you'll have two choices. Try to catch up and die from getting one shot by trash over and over or stay far behind killing trash by yourself.
This is the problem with "group" gameplay now.
But if you are going to run Rathma in groups then you will need to Blood Rush - Metabolism to keep up with your group.