Items can only be added to one group. If you are in the Group/Item tab and do not see an item you would like to add to a group, first check that in your Group Options (select Groups at the top of the group listing), that the "Show Ungrouped Items for Adding to Subgroups" box is checked. If you still do not see the item you would like to add, check the tooltip to see if the item is already in a group. The tooltip will list the group which the item is currently contained in. You can either remove it from this group to add to your group or use the import / export to manually move the item (by shift-clicking into the import box or entering the itemID) to the desired group.