To remove a member from a group you have created press the contacts button, as seen below. This will take you to the contacts page.
Press the right arrow key once to get to the groups menu. Press the softkey more till softkey edit appears, then press it.
Scroll down to the appropriate group member and press the softkey Delete. Confirm your selection by pressing softkey delete again.
Pressing the softkey Done will save the group with the updated members members list.