The team
parameter allows you to target based on the scoreboard team:
/execute @e[type=Snowball] ~ ~ ~ /kill @a[score_Hider_min=1,r=3,team=TeamName]
Alternatively, you can target players that are not on that team (including those not on any team):
/say @a[team=!TeamName]
By leaving the value blank, you can target players not on any team.
/say @a[team=]
And by using the NOT operator with a blank value, you can target players on any team.
/say @a[team=!]
The wiki has a list of target selector parameters here.