If you are the Admin of a group, you are able to Promote other group members to give them Moderator powers. To do this, when viewing the members list for the group you will see an 'Admin' icon in the top right hand corner. Tapping this will enable the '...' icons against the members within the group. Tapping the '...' icon will present you with the option to Promote the group member. Once promoted, you will see a "MOD" badge next to the users name.