As an open-source project, the Readium Foundation welcomes contributions from everyone - from individuals, groups and corporations. The best way to get started with Readium is to read through the introductory materials on this site, subscribe to the mailing list and, of course, get the source code and start spelunking through it.
An important note to bear in mind is that YOU are the Readium team. Readium has no “dedicated team”. Other than a consultant or two and a couple of part-time employees, everything in Readium is done by the Readium contributors - some employed by Readium Foundation members, some simply individual contributors.