Ok, I understand that it isn't supported, but at the moment I can't try other solutions.
And since that aside from that filter, the rest works, I don't want to give up like that.

Infact if I look for the following values (then on the groups)
Search base: cn=groupname,ou=group,dc=pigreco,dc=it
Filter: (member=cn=Name Surname,ou=people,dc=pigreco,dc=it)
I get if Name Surname is part of the groupname group

If I search
Search base: dc=pigreco,dc=it
Filter: (member=cn=Name Surname,ou=people,dc=pigreco,dc=it)
I get the list of which groups Name Surname belongs

But with this (then on the people)

Search base: dc=pigreco,dc=it
Filter: (memberOf=cn=groupname,ou=group,dc=pigreco,dc=it)

I have no result and in the log I read:
get_ava: illegal value for attributeType memberof

obviously same thing with:
(&
    (uid=n.surname)
    (memberOf=cn=groupname,ou=group,dc=pigreco,dc=it)
)

:(

On the "groups" it is works! On the "people" it doesn't work (get_ava: illegal value for attributeType memberof)

It's frustrating!