I was going through my inventory and saw an object that said GoonSquad Party box. What is this?

[Alwin Alcott 9,594]

Well you clearly accepted this object from somebody, so if your last line is correct one of your friends sreads things from goonsquad.

If you deleted everything not much can happen anymore. Normally all possible threats are gone than.

I never heared about demo hair that is grieved, most likely a little rezz problem and your conncted it to the previous things, but really doubt that.

[Alwin Alcott 9,594]

Well you clearly accepted this object from somebody, so if your last line is correct one of your friends sreads things from goonsquad.

If you deleted everything not much can happen anymore. Normally all possible threats are gone than.

I never heared about demo hair that is grieved, most likely a little rezz problem and your conncted it to the previous things, but really doubt that.

[Lindal Kidd 16,054]

GoonSquad is a fairly nasty griefer item.  However, nothing that is simply in your inventory can do anything bad.  You have to wear or rez the item.  It's good you deleted the stuff from your inventory, but check all of your worn items to make sure that something is not attached to you.  Any prim attachment (including hair, though I don't think the demo hair you tried on is the culprit) can contain a script to cause particle, rezzing, and other griefing effects.

[Innula Zenovka 3,439]

I think, though I've never investigated it thoroughly, that items you're offered while you are offline are accepted by default.   That is, when you next log in, the item is there in the "Recent" section of your inventory, and it's up to you to delete it, either by clicking "Reject" on the notification when (if) you see it or deleting the item manually.

Certainly when I receive an offline notification that someone's offered me an object, when I log in I've never had to look for the notification and click accept -- I've always gone straight to my inventory and dealt with it there.

I suspect, therefore, that you were offered the GoonSquad item while you were offline.  That's the simplest explanation, and similar things have happened to me in the past.  

As Lindal says, there's no real cause for concern -- the object did no harm simply by placing items in your inventory, and you and your neighbours are fortunate that's all it did.    

It's rezzing suspect items you need to be worried about, not having them in your inventory where they can do no harm.