If "Enable Group Search First" = "Yes" in Ambari, this feature (group sync) runs in Usersync even if "Enable Group Sync" = NO.
Setting "Enable Group Sync" = "No" hides all values related to group search, including Enable Group Search First.
This caused me two problems:
1) Errors in usersync.log due to invalid values in "Group Search Base". Removing this and adding a space for the value got rid of those errors.
2) No users synced even after fixing the errors because the group search failed to find users, even though group search was disabled via "Enable Group Sync" being set to No.