You don't add groups per se.
The LDAP strategy defines the search path in the LDAP heirachy from which Splunk will search for groups.
If you add a new group to AD, as long as that group is in your Strategy's search path, the group will appear in your mapping options to allow you to assign a role to it.
Since your posting this question, I presume you have added the group to AD, but cant see it in Splunk?
If so - try adding the group into the same OU as your existing Splunk groups, or redefine your strategy to search wider in your domain.
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