While the need for LDAP integration and OpenVPN seems straightforward, it seems to me that the documentation for the auth-ldap plugin is not very easy to locate and find. Take for example the following auth-ldap.conf configuration file

<LDAP>
  URL ldap://ldap.example.com
  Timeout 15
</LDAP>
<Authorization>
  BaseDN "ou=users,dc=example,dc=com"
  SearchFilter "(uid=%u)" # (or choose your own LDAP filter for users)
  RequireGroup false
</Authorization>

This is a very handy starter that would allow any user with a working password under the ou=users part of your tree to be granted access. But what if you would want to restrict access based on group membership? According to fragments of documentation scattered at different bits of forums and StackOverflow / ServerFault, you’d need to set RequireGroup true and then use the BaseDN of the group and the memberUid attribute within a <Group> ... </Group> subsection of Authorization. This never worked for me. What worked was changing the Search filter to include group membership:

<LDAP>
  URL ldaps//ldap.example.com
  Timeout 15
</LDAP>
<Authorization>
  BaseDN "ou=users,dc=example,dc=com"
  SearchFilter "(&(uid=%u)(memberOf=cn=openvpn,ou=groups,dc=example,dc=com))"
  RequireGroup false
</Authorization>

Voila!

I did not come up with this. I found it via random Googling somewhere in SO (I cannot remember and cite that answer anymore).