Forum Discussion
Office 365 Groups & Spam
And even though we've repeatedly tried to bring this issue to Microsoft's attention (I know I have spammed few people several times), there is no supported option to check on those messages. Best you can do is run a message trace or get the folder stats as Tony suggested. But for actually getting the message, things are trickier - you either have to play with mailbox searches or use unsupported workarounds such as opening the Group as additional mailbox in OWA.
I suppose that if you really wanted to eliminate messages held in the Junk Email folder of a group mailbox, you could run a content search limited to the folder id of Junk Email and then execute a soft-delete action against the results of that search. All in PowerShell, naturally [see chapter 19 of "Office 365 for IT Pros" if you want to find out how to do this kind of thing...]
Otherwise, I would simply leave the items to rot in the Junk Email folder. They don't do any harm there because no user can get to them.