First off all, gankplank can get quite a lot of assists, without even leaving his lane. After level 6, start watching the map, and as soon as you see your teammates engage, or your opponents engage on your team, Cannon Barrage on the enemies, try to place your ultimate so it slows the enemies for as long as possible, or your teammates might not be able to get the kill. Also, apologise to your team mates if your ultimate steals the kill, even know it was your intention, it is kind of unfair since you mostly just slowed, and your team mates might get angry with you.
In teamfights, you should start by poking your opponents down with your Parrrley even before the engagement, this might force them to back off, or initiate on you with low health. Make sure to walk as much as possible in between Parrrley so that you get the Statikk Shiv passive on them :). When the fight actualy breaks out, start off by getting in range for your ad carry to get your Raise Morale active buff, every bit counts. Then focus down the enemy squishies, and if your opponents start to run, or your teammates start to lose, throw down Cannon Barrage for the slow, and maybe even enough damage to kill someone.
A gankplank pentakill :)