Well, I don't know what your plans about Harry might be. Sirius, once he sees Harry, will want to get his memory back. He'll probably respect Harry's desire to not know about his past (if that's how Harry feels about it), but he'll also want Harry to know a few things (that he was a hero, about James and Lily, and that Sirius is his godfather and is looking out for him) that will make him conflicted.

Sirius can find out anytime (it doesn't affect my plans for him), but when he finds out he'll probably try to push some of the truth on Harry, so when you want Harry to learn some things about his past should impact your decision about their meeting (although of course I'll respect what you do and don't want Harry to know). I'd prefer for them to meet by chance, because Remus knows about Harry and Sirius will actually be less mad at Remus if he finds out Remus has been hiding Harry from him after he's had a chance to get used to Harry being there (he currently doesn't know what happened to Harry, but suspects that Harry is dead).

If you want Harry and Sirius to meet sooner rather than later, then Harry should come to the bbq, and they can meet there. If you want to postpone that meeting, than Harry might not want to play a prominent role in the bbq, since Sirius is already there (although he does have a lot on his plate right now between Snape and Remus...he might just fail to spot Harry in the crowd).

Basically, whatever works best for you. :)