Add users to groups with ldap

I need to add an existing user to an existing group with my new ldap setup, which is running on a Debian lenny server. Here's what I've got for an ldif file:

dn: cn=johnstones,ou=Groups,dc=newcrossroads,dc=net
changetype: modify
add: uniquemember
uniquemember: cn=jess

and here is my command and output:

robert:~# ldapmodify -x -D cn=admin,dc=newcrossroads,dc=net -W -f /var/tmp/adduser.ldif
Enter LDAP Password:
modifying entry "cn=johnstones,ou=Groups,dc=newcrossroads,dc=net"
ldap_modify: Object class violation (65)
additional info: attribute 'uniqueMember' not allowed

I've also attempted to do the job with ldapscipts with the following results:

robert:~# ldapaddusertogroup jess johnstones
Group johnstones not found (or jess already member of johnstones)

That last in particular confuses me; the group exists and every attempt I've made to add the user has failed. So two questions:

How do I write a proper ldif file to add a user to a group?

How do I view group membership?

Please bear in mind that I really don't know what I'm doing. I got ldap running more through good luck than good practices. Unfortunately the online guides I've found assume that you do know what you're doing, and I've been fighting with this for three days. Can anyone help?