Allow users to categorize mails (and maybe also files) in a group with color flags like in regular mailbox. It would make the adaption easy.
When we look from a user point of view the functionality in the different should be similar. Whether it is in a mailbox, an archive, a shared mailbox or a group, the same actions to organize (follow up, categorize, move...) for a user should be available to avoid confusion. Today we notice that users are reluctant to switch to a group because basic functions are missing to get organized amongst a team.