I'm going to ask about people's fitness habits in order to collect data for my statistics class
Why are you focusing on their fitness habits?
Everyone is so overweight in this group, perhaps it's time we should be thinking about working out
You shouldn't concern yourself with the weight of your group members. Their weight doesn't have anything to do with their health and you shouldn't expect them to disclose personal information to you.
They want me to know about themselves, I'm sure about it.
If they wanted you to know that information, they would probably share it. It's not polite or appropriate to ask other for personal information. Would you want someone to ask you personal things without your consent?