There is no “fee” for installing or using the app and you don't need to pay a subscription to register or use the app at this time.
You only need to have a number of points in your balance for specific actions such as when you want to send a direct message to someone for the first time or other specific actions offered within the app. You will always see a pop-up to let you know when an action requires points or to have an active subscription.
Depending on the person you contact, you may also have to pay the price that they want charge for their time, products and services. This is something that users manage directly between themselves and Instantgo is not part of those transactions.
You can also decide to charge for your time and knowledge but will need to be subscribed in order to do this. You will then simply need to set the different rates you want to charge and add the products or services you want to offer.