If you are the owner of the group, you are not able to simply remove people from your group created. You only have the power to delete the whole group.
You can find the Delete Group button under settings.
Note: If you want to open a new group, a group name as well as a group image can be applied. Moreover, you can decide, which participants you want to add, with the certainty that no former group participant can join.
If you want to leave a group, just click onto the trash can icon. You'll find this icon on the upper right corner of the group.
Due to this action, your user avatar vanishes from the owners’ board. Therefore, the initial group has not been deleted by yourself, you only removed yourself.
Summarily it is a simple concept. If you have added a person to your group, only the invited person can delete itself from the group. If you want to get rid of people with whom you are sharing a group with, delete the whole group and create a new one.
Everything clear so far? 😉