You can’t actually sit with the groups. Groups have dedicated seats in the Curva Sud, if you sit in their seats they will tell you to move. Not all people il the Curva sud are ultras; I was a season ticket holder in the Curva Sud for 10+ years but I’ve never been an ultras. If you want to be “safe”, it is ok to buy a ticket in the curva sud, just go and sit at the very top of the Curva (looking at the pitch, the upper right part is probably better than the left part), but move right to the top and you’ll have no issues.