How to add selected users to OG Group from field selection when creating group
This is the rule I created to solved it:


After updating/adding existing content of type 
Entity has field Parameter: Entity: [node], Field: FIELD WITH UNAMES
Crete a LOOP
(In Loop) Subscribe user to group -- Parameter: User: [list-item],
Group: [site:og-context--node]


Here is an export of my rule:

{ "rules_watch_list_group_add" : {
  "LABEL" : "Watch List Group Add",
  "PLUGIN" : "reaction rule",
  "OWNER" : "rules",
  "REQUIRES" : [ "rules", "og" ]
  "ON" : { "node_insert" : [], "node_update" : [] },
  "IF" : [
    { "entity_has_field" : { "entity" : [ "node" ], "field" : "field_watch_list" } }
  ],
  "DO" : [
    { "LOOP" : {
        "USING" : { "list" : [ "node:field-watch-list" ] },
        "ITEM" : { "list_item" : "Current list item" },
        "DO" : [
          { "og_subcribe_user" : { "user" : [ "list-item" ], "group" : [ "site:og-context--node" ] } }
          ]
        }
      }
    ]
  }
}