Agreement technologies have settled the basis for creating systems that operate on the basis of agreements in societies of independent, autonomous computational entities (agents). However, nowadays more and more systems of such kind rely on a seamless interaction of software agents with humans. Humans work in partnership (directly or indirectly) or closely related with agents that are able to act autonomously and intelligently. Specifically, humans and agents have the ability to establish a series of relationships/collaborative interactions with each other, forming what might be called human-agent teams to meet their individual or collective goals within an organisation or social structure. Systems in which people and agents operate on a large scale offer an enormous potential but also require the consideration of additional issues. In this paper we analyse the open issues that may be addressed for researches in order to develop open human-agent systems. We present a real-world case study and an abstract architecture proposal for such systems.