To edit the members of a group, 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.
Pressing the softkey more will make the softkey add appear, press it.
This will then bring up the below screen, either search for someone or use the arrow keys to find the appropriate person. Highlight the person you want to add and press the softkey Select. Once you have finished adding in the members press the softkey done to finalize member selection.
Note: When entering letters you will need to press the button multiple times to get the other letters displayed on the button. Capitals and symbols can be selected via the second Soft key
Pressing the softkey Done will save the group with the new members.