You want to make use of the ldap.searchfilter setting. Your filter will be specific to your directory, but it should be something along the lines of:
(memberOf=cn=spark_groupname,cn=OU,dc=domain,dc=tld)
This is very basic and can be expanded upon quite a bit. Fortunately if you need a more detailed example AD seems to be the most common form of LDAP people are using with Openfire. A quick google search turns up lots of examples with much more detail than I can provide.