I have two private accounts, 3 accounts for work and 3 for my own business. It would be great to be able to create groups. Outlook for Mac already has this functionality and works great. Outlook also has the ability to show all groups at once, if you want to.
Maybe you could create a drag function in the account settings window?
You can’t group them, sorry.
But you can change the order. Just go to Menu > Accounts and select one and use the arrow icons to move it up or down. The order the accounts appear there, will be the same order they appear in the folder list in the left sidebar.
That isn’t happening when I try!
Without opening any options etc, just drag the inboxes up / down.
However, this is not as good as the similar options in Postbox (which I have just moved from) as inboxes could be grouped, then collapsed.
Having a disability affecting my coordination, I am glad to have had to move to eMc as the staff at Postbox have a really bad attitude towards accessibility, especially scrollbar accessibility.
eMc staff are the complete opposite 
Sarah