I've had three group interviews/assessment days and lots and lots of one on ones/two on one. I've been successful most of the times.
Group assessments are less formal (especially for retail jobs and stuff like that) and they're most likely going to give you simple tasks to see how confident you are with new groups of people, coping with deadlines and how well you can speak to others and help out them out. Try and get involved as much as you can and try to get more quiet people involved too (like ask them what they think etc).
For one on ones, research the position, company information and type of interview questions online. Always make sure to shake their hands confidently (no limp wrist), maintain eye contact and smile. If you are not good at selling yourself (I can't
) just keep trying to relate what you're saying to any past experiences that you have had. For example if they ask you about dealing with stressful situations and customers, talk about a time that you have dealt with it because that shows that you can handle yourself and that you aren't all talk. Just make sure that you did the right thing though (for example listening to them, keeping calm, speak to the manager etc)
Hope I helped
