You have a team with great potential to be some hybrid of sweeping and wallbreaking.
Open with Foretress, set up Rocks, Sandstorm, or whatever, and explode out or switch. Depending on the current enemy, send in Starmie or Charizard to sweep. Eventually, this will likely lure and Electric-type to come out and KO one of them. This is where Electivire comes in.
Switch Vire in on an Electric-type attack, and activate Motor Drive. Continue with the sweep. From here, one or two Pokemon will be fainted, so continue to switch between your attackers and KO opponents. Explode with Fortress to break walls, and use Umbreon to break down an enemy.
By now you'll have killed off five Pokemon (on average), So use whatever's left to go all-out and win.
The free turn you get for Electivire, Umbreon's stalling, and Foretress' hazards and exploding are important parts of this strategy. Use or modify as you see fit!
To answer your question regarding your Pokemon's roles:
Charizard - Could be a mixed sweeper or Special sweeper, meaning it can attack with Special attacks purely or Special and Physical attacks.
Starmie - Special Sweeper through and through.
Umbreon - Stall / Wall, because of his Defenses and ability to survive.
Torterra - Physical Tank, since it's got HP, along with good Physical stats (Atack / Defense).
Forretress - Lead; best to begin with Forretress. It can set up Sandstorm or Stealth Rock from the beginning to hinder the opponent.
Electivire - Mixed Sweeper. One Motor Drive Boost makes it great for attacking with both Physical moves like Cross Chop, and Special moves like Thunderbolt.