You can only guest into the lounge if you have BA/Oneworld partner status (Oneworld Sapphire or Emerald). A Business ticket alone doesn't give you any guesting privileges so status is important if you're intending on guesting someone else in. Plaza Premium or Aspire lounge passes can be purchased for the respective T5 lounges if you don't have any.

As a QR J passenger you use the Aspire Lounge in Terminal 5. If you have Oneworld status you can use the BA lounges in T5A. There's also an additional Galleries Club in T5B which is usually quieter and more likely to be nearer to your departure gate in T5B or T5C however don't go over there without checking the departure gate as it isn't straight forward getting back.

Pyjamas and QR specific service questions are probably best asked in the Qatar Airways forum. From memory they're only offered exclusively on night flights or flights timed longer than 10hrs.

Edit:
I see you already asked the same question there earlier this year and got a response confirming PJ's and amenity kits for those particular flights.