The portal is drawn, and based on your choices when making the portal it puts the header of each column you wish to show in the first portal row. The portal then shows each entry in that table through that portal row, all the information for each column you had on the top, first portal row in layout mode. There is no need to group them.
Usually i find it best to make portal header text with a transparent background then I can make alternating colors in the portal rows for easier reading.
"it puts the header of each column you wish to show in the first portal row"
How do you do that? I've always had to use the layout text tool to add the field labels (column headers) after the fact and located just above the portal...
BTW, we are not limited to that "table view" like arrangement of fields in columns. We can also resize the portal row to change it into a "mini-form" view of the data as well.
@Phil - good point on the BTW