how do I add an existing user to a group in ldap(ibm secure directory) using jndi?

I tried using this code:

mods[0] = new ModificationItem(ctx.ADD_ATTRIBUTE,
new BasicAttribute("member", "cn=user1, ou=company, o=ibm, c=us" ));
ctx.modifyAttributes("cn=group, ou=company,o=ibm,c=us", mods);


but user1, is not beign assigned?
can anyone let me know where I am going wrong, and how Can I solve this problem?