Network slicing has recently appeared as one of the most important features that will be provided by 5G networks and is attracting considerable interest from industry and academia. At the wireless edge of these networks, most of the contributions in this area are related to cellular technologies leaving behind WiFi networks. In this work, we present a resource allocation mechanism based on airtime assignment to achieve infrastructure sharing and slicing in WiFi Access Points. The approach is simple and has the potential to be straightforwardly used within scenarios of wireless access infrastructure sharing.