How do I...Give specific rights to a set of LDAP accounts?

First you'll likely want new accounts to have no or limited access. Do that by adjusting the security/new-account-role setting as described here. Then you'll want to put all of your users that you'd like to have contributor access into an LDAP group such as "DekiWiki-Contributors".

Using the control panel/group management screen, you'll want to add this group and give it the Contributor role. This way your users have a role None (and thus no access) but users in the DekiWiki-Contributors group inherit the contributor role.