If the Active Directory security groups that are created during the initial install of Exchange 2013 get removed unintentionally, they can be recreated by running the setup /prepareAD command. This is pretty well documented, but be sure to use the setup files from the most recent version of Exchange installed in the environment. If the Exchange environment is made up of mixed builds, always use the most up-to-date build for running the setup /prepareAD command. Cumulative update builds count as well, not just service packs.