Every current party member is affected, at any range on the map; members exchanged into the party and summoned creatures, will not have it until re-activated. On entering the camp, all party members will lose the effect except for the Warden if he/she is the one who activated it.
On the consoles Song of Valor provides 2x the stamina/mana regeneration.
The stamina/mana regen bonuses are determined when the bard begins singing the song; changing equipment while the song is active does not change the stamina/mana regen bonuses.
The real benefit is not that it gives the bard a big stamina charge, but that it gives all 4 party members a little mana/stamina charge. However, the charge is so small, to regenerate +0.5 mana/stamina points per second one would need 60 cunning that works out to 1 or 2 extra ability uses for everyone.
This ability will likely be of no benefit to the rogue. A rogue with 40 cunning will require fights to last more than two minutes before getting any net benefit, as it will take that long with the slow regeneration rate just to recoup the 50 stamina cost of sustaining it.
Switching it on after expending that 50 stamina on a couple activated abilities you gain the benefit for the entire party at the minimal cost of 5% fatigue.
This ability synchronizes well with a double-Haste strategy; the extra ~+1 mana regeneration can help to ensure that Haste can be used on both casters without draining mana constantly.