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 "MindTouchContributors".

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 MindTouchContributors group inherit the contributor role.