Data activism has emerged as a response to asymmetries in how data and the means of knowledge production are distributed. This article examines MyData, a data activism initiative developing principles for a new technical and commercial ecosystem in which individuals control the use of personal data. Analyzing material collected at a formative event shaping MyData activism, we examine how more just data arrangements are framed to enhance equal participation. Our analysis shows agreement on what is ultimately at stake: individual data agency and fair competition in the data economy. However, two alternatives are offered for what participation involves. Collaboration with commercial actors favors framing participation as agency in data markets, thereby potentially limiting the scope of what is at stake. The alternative framing presents a rights-based understanding of economic and civic agency, potentially leading to a broader understanding of participation in a datafied society.