configuration -> host groups -> open group
configuration -> host groups -> edit group

In this case, if we move a host from the "hosts in" box to "other hosts" with group selected, it seems as if the host will be added to that group, which is not true. the host is still displayed in that list - until we switch to another group, then back. in such a case also it's pretty hard for the user to figure out where did the host go, unless user knew before to which additional groups host belongs to.