Hola Ama

In that same lesson there is no mention of written accents. The reason why the imperative has a written accent on the í at the end is because the stress is on that last syllable. Generally, when the stress of a word falls on the syllable with a diphthong like this one: "sonreíd" the stress is on the strong vowel (in this case e) not the weak vowel (i), but with all infinitives, as you can see in "sonreír" for example, and the imperative for vosotros ending in -ad, -ed and -id you stress it on that last vowel. If there is no written accent on the last í then the word will not be pronounced as it should :  sonreíd, but instead you'd have to pronounce it as sonreid. 

There is a lesson about accents on diphthongs (bear in mind it's also necessary to know about rules of accentuation). 

I hope this clarified it.

Saludos

Inma