I feel sad for our friends who got caught in between me and my gf’s break up. Technically, they were her friends until I got introduced to them. We’ve made our own bond and I now consider them as my buddies too. I’ve talked to one of them recently and he said he missed the Sunday bbqs at each other’s houses. He said he wished we could all all hang out and none of it (our break up) would matter.

I don’t like the feeling of burdening people with the events of my life. I told him we could all still hangout if she’s (my ex) already comfortable with it. I think the last time we all hung out was during the new years party dating service celebration we had early 2018. So my friend said he’ll plan a day for us to get together again. Was I wrong to say it was fine to hangout with them? They were her friends first not mine.

These types of scenarios are often complicated. You can't really say until you're there. You won't know if you made the wrong decision unless you try. So if you think it's worth it, why not take the chance to bond with your friends? 

It depends on the type of friendship that you have with those friends of your ex. If you feel very comfortable with them and that hanging out with them is just between you and them that means topics about ex are excluded then I think that's just fine. One more thing, you only hang out when you enjoy their company. And now that you have parted with your ex, do you still like to hang out with them just like the good old times? If you are not comfortable in being with them then it is easy to think of an alibi to evade their invitation.

I find it hard whenever stuff like this happens. I find that talking to them helps. People are a lot more mature than we think. They'll understand whatever your decision is. Parting in good ways will be nice for you and your friends. But if you've chosen to stay connected, knowing what you're going through will help them gain much needed context.

This is really complicated, but I think there is nothing wrong to stay friends and hang out together, still, I don't think that your ex-girlfriend will adopt this behavior especially if you will come with another woman or she will come with another man.
I think is better to hang out with her/your friends when she is not around.