In an imperative sentence, you have to use the root verb by dropping off "me-" or "ber-". The tip for the previous command lesson explains some sample sentences. https://www.duolingo.com/skill/id/Command/tips-and-notes
I also threw a similar question as yours on HiNative. https://hinative.com/ja/questions/18066042
If you add "ber-" or "me-", it isn't regarded as an imperative sentence and grammatically incorrect, according to a native speaker.
Furthermore, the suffix "-kan" has five roles, only one of which is to convert from a verb to a noun.
One of the other roles of "-kan" is to add the nuance of "to do for someone's sake" in an imperative sentence. -- This explanation is based on a grammar textbook issued by an established university (sorry it's written in my native language Japanese... but at least I can guarantee you that this explanation is solid). http://www.coelang.tufs.ac.jp/mt/id/gmod/contents/explanation/304.html
For example: "Biarkan anak-anak itu bermain." (Let those children play.) https://forum.duolingo.com/comment/28989050
"Biar" is a verb and adding "-kan" with "biar" suggests "letting kids play for their (or their parents') sake".
One of my dictionaries translates "bersama" as not only "together" but also "simultaneously". So, I asked native speakers about the meaning of "bersama" in this sentence. https://hinative.com/questions/18160048
If you say "simultaneously" (i.e. Don't work on more than one task at one time), you would say "secara bersamaan" instead of "bersama".