Perhaps you'd prefer your calendar users' rows to be organized alphabetically, or perhaps by seniority, or perhaps by some other method altogether. You can easily rearrange those rows right from the Calendar Tab itself.
To alter a user's positioning on the grid, click the small cog/gear icon located in the top-left of any of the groups (which cog/gear you click is arbitrary).
You will now be able to click & drag user rows around to reorder them. Once you have everyone in the order you prefer, find the area where you first clicked the cog/gear icon, and click "Done Ordering", which will save your changes and propagate them through calendar periods.
NOTE: If you drag someone to a different group altogether, this actually changes their group assignment from that date forward, just like if you were to go to the "Assign People to Groups" page and re-assign them there.