You can group things in two ways:
Open the Sample viewer, add an attribute to the table, e.g. called health and then give each patient either a healthy or sick value (or yes/no 1/0, whatever)
Then you can select the attribute column and use the group by attribute menu item to define two groups.
However, these groups only really play a role in the PCoA analysis.
You can't use groups in the charts viewer.
The other way to group samples is to use the Samples Viewer, select an attribute column and then select
the Compare Relative or Compare Absolute menu item to create
a new comparison in which each "sample" is the union of the input samples, in your case healthy vs sick