To be honest, there is no certain preparation. It's more about understanding the role you applied for and thinking around possible challenges associated with it and that's it. The assessment is always tweaked to match your job role. I went in for a BA role and that would be different from what you are going for. In group sessions, don't be domineering...contribute, give others room to speak before you, be confident and assertive...open to suggestions and take the lead if you can but be careful not to overplay your hand. Have a good understanding of strength based interviews and ample examples with it