Create a dynagroup (group definition) and compute its resulting group(s), then go to the group and its inventory tab. Note a button "Update membership..." at the bottom. This button serves to add members to the group. This makes sense for classic comp & mixed groups, but not for DynaGroups, as DynaGroups are calculated (=managed) by their definition. Any manual addition will be cleared by a recalculation. So that button should be inactive for groups based on a group definition.