Below are screenshots showing how to add a group and its members. Follow the "Contact" - "the PLUS icon" - "Add a group" - Enter a group name - Hit the one you want to add to the group in the Contact list on the left.
Note: When you have hit a Contact to invite him to the group you have established, a invitation message will show up in his "New group" tag besides the Contact tag. He can choose whether to joint or not, but before he reject to be jointed, he can still receive emails you send to the group. Only after he reject to joint the group, will he not receive email sent to the group.
If he agrees to joint, the new group will become his Contact and be listed in the category of "My group" in the Contact tag.
A group should have at least 3 members while too many members will lower the speed.