The option to group objects is not available anymore in Prezi Next. Why? Because there is no need for grouping, now you can use the fade in/out effect on multiple objects simply by selecting them.
Select multiple objects by holding down the shift key and clicking on objects or dragging over them. Now enter the animations menu and the fade in or out effect can be applied straight onto the selected objects. This eliminates the need for grouping. The only downside of dropping the group feature is that if you need to move a lot of content groups simultaneously while designing your presentation then you constantly have to select your items again. This can cause some trouble when your Prezi has many design elements, but overall I think Prezi made the right move.
The grouping feature in Prezi Classic sometimes caused even more pain, because if you accidentally grouped an object that you did not want to, then you had to ungroup the whole thing and it could get quite messy. The non-grouping way of animation seems a much better option, because now you can move each of the objects separately and edit texts while still keeping the animation.