Default user relationships with the site admin
I was able to achieve this with rules + user relationship rules modules.

Export:

{ "rules_follow" : {
    "LABEL" : "follow",
    "PLUGIN" : "reaction rule",
    "REQUIRES" : [ "user_relationships_rules", "rules" ],
    "ON" : [ "user_insert" ],
    "DO" : [
      { "user_relationships_rules_action_create_relationship" : {
          "relationship_type" : "1",
          "requester" : [ "account" ],
          "requestee" : "1",
          "approve" : 1
        }
      }
    ]
  }
}


Rule Details:
Event: After saving a new user account

Conditions: None

Action: Request, create or approve relationships between users

Rule Settings:
Relationship Type: [your defined relationship] (follower)

Requestor: (Data Selection Method) Data selector "account"

Requestee: (Direct input mode) "1" (or the user ID of your admin account)

Approve Relationship: TRUE