Normally any app is accessible to your family after setting up Family Sharing, inviting your members to your Family Group, and successfully accepting the invitation. A family group has the limit of six members. https://support.apple.com/en-us/HT201088 ; also from my experience all the elements of the family group have to be in the same country store.
The app has then to be selected using your Account in the Purchased section by your family members (as aidan posted).
However, there are hidden caveats. Not all apps can be shared.
As it can be seen perusing this document, https://support.apple.com/en-us/HT203046 an app can be marked as non-shareable and furthermore, an app bought by an Apple ID account for Students is not shareable to the family group.
In-App purchases are also not available for Family Sharing. From http://9to5mac.com/2014/09/19/ios-8-how-to-set-up-and-use-family-sharing/:
In-App purchases are not supported by Family Sharing. So if your children are playing games with the pseudo-currency, or if they are playing games that require in-app purchases to get to the full version these you would be paying for twice. Most apps are now compatible for Family Sharing. However, developers do have the option to opt out of enabling their apps to be used with Family Sharing. When you are viewing the description of the app in the App Store, there is now a label that will say Family Sharing. If it says yes, that means the app is compatible with Family Sharing and can be redownloaded by other family members, before the existence of Family Sharing.