Add role to user after joining organic group with rules
Now your Rule runs after user requests membership. But you should run it after OG membership was approved.
I prepared Rule which adds role “member” after membership was approved or if it was added by admin (auto-approved). Here is export (4 is id of my drupal role “member”): 

{ "rules_after_add_user_to_group" : {
    "LABEL" : "After add user to group",
    "PLUGIN" : "reaction rule",
    "TAGS" : [ "test" ],
    "REQUIRES" : [ "rules", "og" ],
    "ON" : [ "og_user_insert", "og_user_approved" ],
    "IF" : [ { "data_is" : { "data" : [ "og-membership:state" ], "value" : "1" } } ],
    "DO" : [
      { "user_add_role" : { "account" : [ "account" ], "roles" : { "value" : { "4" : "4" } } } }
    ]
  }
}


Screenshot:
 

Tip: you can enable Rules debug (page admin/config/workflow/rules/settings) to see when your rules are executed.