How to automatically add new users to a OG Group (MyCommunity) using Rules?
One of the elements you can use under actions is "Fetch entity by id". Say the node id of your MyCommunity group is 10, then the Entity type value will be Node, and the Identifier value is 10. In the Provided variables section, you give your fetched entity a label and variable name. Let's give it a label of MyCommunity, and name it my_community. Save the action element. The next action element to add is "Add entity to group". The Entity data selector is the new user so give it the value of "account". Then under group, the data selector is the variable name we created in the previous step, which is "my_community". I tested this out locally and it worked. Here's an export of the rule.

{ "rules_115407" : {
    "LABEL" : "115407",
    "PLUGIN" : "reaction rule",
    "OWNER" : "rules",
    "TAGS" : [ "stackexchange" ],
    "REQUIRES" : [ "rules", "og" ],
    "ON" : { "user_insert" : [] },
    "DO" : [
      { "entity_fetch" : {
          "USING" : { "type" : "node", "id" : "10" },
          "PROVIDE" : { "entity_fetched" : { "my_community" : "MyCommunity" } }
        }
      },
      { "og_group_content_add" : { "entity" : [ "account" ], "group" : [ "my_community" ] } }
    ]
  }
}