Sorry, but I don't know how to do that. I tried many different things but the best result I could get is that for every person in People a group is created which is named like the location. So, ... Paris would be present about 50 times meaning I get 50 Groups that are named - nothing! Nothing because the location name appears several times and it seems that Openfire does not like this very much.
Is it really possible to gather members and add them to a group by means of a ldap filter? It seams to me that Openfire expects a group or several groups in LDAP that contains a list of all users - prepopulated!
If it is possible to get a group only from people entries - without any prepopulated groups - then please enlighten me how I could do this via ldap filters because as far as I got, this does not seem to be possible.
Nothing worked. So... I used mailing lists Groups. Not pretty but it does the job.
A warning / advice:
When you use a full email address as username... you have to exchange the @ in the user name to \40 when you want to login with Pidgin. Otherwise it is not a valid XMPP id.