Product configuration is a key activity for enabling mass customization. It corresponds to tailoring a software application from a software product line, respecting individual customer requirements. In practice, the product configuration activity is challenging, mainly because it involves numerous stakeholders with different expertise making decisions. Several works claim to decompose the configuration activity into pre-arranged stages, whereby stakeholders can make their decisions in a separated way and coherent fashion. However, due to the decentralization in geography of the knowledge required in each stage and that customer requirements could be imprecise and change frequently, the proposed solutions do not apply. To address these issues, this work proposes a dynamic and collaborative product configuration approach based on the personal assistant metaphor. Personal assistants in a cooperative way coordinate stakeholders´ decisions and proactively perform tasks, such as reasoning about the ramifications of decisions, integrating distributed decisions and resolving divergent requirements. A tool prototype, called Libertas, and two case studies that evaluate the applicability of our approach are also presented. The first case study analyzes the configuration process to configure an operational system to support the business requirements of an enterprise. The second one addresses a scenario of a software product line for web portals.