When placing an armor stand and running this command:
/scoreboard teams join Main @e[type=ArmorStand]
Main being the team, the console says it successfully added the entity to the team.

However, when running this command in a command block:
testfor @e[type=ArmorStand,team=Main]
The command block does not output a comparator signal, and there is no text in the "Previous Output" section of the command block.

After adding the entity to the team, the command block does output a comparator signal if testing for [team=!Main]. It appears as though the entity is just not being added to the team.