If you want to be useful for your team, you should remember several things:

1) burst team's XP
quick push 1st line towers, fill all empty lines (use 'Q' on creeps at this line)

2) annoying your enemies
split push. If you see, that your enemies pushing one line - be in another one. You can send 'R' to top, and sitting in the bush on bottom line, for example, and use 'Locust Brood' talent every cooldown.
Promp You should feed your Monstrosity first. You can make 15-20 stucks, withdraw him behind your creeps, leave him and focus on your teammate
Thereby you forced your enemies to step back.

3) defend your bases.
Use Mule and shield with 'Q' at your base all the time. You should protect your base and slow down your enemies.
Try to set your 'Toxic Nest' before enemies creeps will come.

4) Always be in all battles.
Assist your teammate as much as you can.
Allies should not be the impression that they are playing 4v5.