when do u call somone dono
 

what do they mean with dono

When they are your boss/important people, I think.

I would not use dono; it means something like "lord". miyabi calls kaouru this, in the anime ai yori aoshi, but it's a special situation. stick with -san, or -sama, with highly respected elders.

殿 【しんがり; との; どの】 (しんがり) (n) (1) rear; rear unit guard; (2) anchor (man); (との) (n) feudal lord; mansion; palace; (どの) ; (pol) person; Mister (mostly in addressing someone on an envelope); Mr; SP

The kanji for "dono" can also be pronounced as "shingari", "tono" and "dono".

Right now, when addressing anybody among my group through email, I/we basically use the word "dono" or "sama", but when talking directly its just "san".

I am not sure on this, but I think its a "kaku-kotoba" or use only during written form.

When his last name is gurai

This honorific is no longer used in spoken japanese. it's outdated. it's only used for addressee on an awards, a public correspondence, a bit of business/official letter or other very formal/public situation.
dono has about intermediate respect between sama and san in the present day.it was used toword norbles, loads, masters, meant person who lived in palace.
in anime, a character like a samurai or an archaic woman uses dono for san.
if anything, it's gravely saying than polite saying.