I've recently played IO(Wisp) and I feel it's good to share my experiences with him. Despite not being able to kill any mobs with particular ease, he is still very much capable of making the difference between a win or a loss.

First and foremost, you need to understand your role. Your aim is to not farm, but to keep your team alive and full of mana through the use of Tether's regeneration. In order to do this efficiently from the start of the game, get a bottle ASAP.
If you get a Bounty Rune (or both) then make sure you buy Courier and a Bottle – otherwise just save up for a Bottle and buy the Courier later.
What you want to do next is go to middle lane, the reason for this is so you can get faster experience than going top (bottom lane is also an option, but that one's best for a solo hero). Your team might complain, or tell you to leave, or even disconnect without any second thoughts as to why a Wisp with a bottle is in their lane – but your experience is much more important than theirs at this point. Once they realize you're stopping them from dying thanks to your Tether Bottle charges and your Overcharge, they'll start to consider you a welcome sight.

From there, you want to be constantly moving in between lanes, the rune spots, and pool while being efficient with your Bottle Charges. The easiest way to do this is to use the two creeps that spawn in front of the Ancient as Tether jumps so you can get around faster.
Once you get in to the swing of things, you'll be zipping back and forth through the base, keeping your team alive and farming. If they still manage to die, then convince them to play passively whenever you're moving about.

This is your early and mid game. Your goal is to keep your team alive so they can farm sufficient items and eventually push out to take towers. Along the way, pick up a soul ring and maybe Arcane Boots – however you'll want to save your gold.
When your team starts pushing out to take towers, that is when you will have your largest amounts of gold income. You need this tower gold in order to build your first big item, Heart of Tarasque. It goes without saying that this item is what makes up for your lack of dealing any damage since you can turn whoever you Tether to in to a DPSing Tank.
From there, you'll want to build more tanky items such as Bloodstone, Skadi, Octarine Core, Power Boots, etc. The goal is to maximize your health pool so that you can sit in the middle of creeps with your team and tank the damage.

Now, things might take a turn for the worst and your team isn't entirely capable of fending off the waves that start to spawn around the 25 minute mark – but worry not, this is where Wisp's second amazing capability comes in to play. In between waves of fighting off creeps attacking your Ancient, you can take your highest DPS carry (who will hopefully have at least one form of evasion or anything else that will help with knocking towers down without dying) and then Relocate in to the Dire base to knock down towers. Since Relocate will have a lower cooldown (even more with an Octarine core) you'll be zipping back and forth really quickly, eventually winning the game should you time it between waves efficiently!

For a skill build. getting your first one point in Tether is mandatory, but leveling it beyond that isn't that useful so should be saved for last. Your E makes you and your partner tankier and lets you output more damage so that gets first priority for maxing. Your W lets you deal some extra damage to creeps, maybe even scoring you bonus gold, so you max that next. Finally, take your ultimate as it becomes available.