Google Play, formerly known as Android Market, had emerged as a major actor on the market of smartphones' applications. Its success depends on contributions of various developers and on their interactions with early adopters in forums. Contrary to previous research that mainly focused on the individual features of developers, this paper puts the emphasis on the collective dynamics of the induced co-production. Content analysis of 184 threads promoting members' applications in a French newsgroup sheds light on the role played both by creators and followers of topics, and on interessement techniques enhancing contributions and applications' development efficiency.