Each user needs to have the SambaSamAccount objectClass added and the sambaSID. Here is the ldif file I used to do this:
[root@gray openldap]# more addsamba.ldif dn: uid=mheintz,ou=people,dc=gray,dc=uchicago,dc=edu changetype: modify add: objectClass objectClass: sambaSAMAccount - changetype: modify add: sambaSID sambaSID: S-1-5-5000
Note that the sambaSID is the same first three fields (S-1-5) as the sambaDomainName sambaSID. The last field is found by multiplying the uid by 2 and adding 1000.