To cite JM in publications use:

  Dimitris Rizopoulos (2010). JM: An R Package for the Joint Modelling
  of Longitudinal and Time-to-Event Data. Journal of Statistical
  Software, 35(9), 1-33. URL https://doi.org/10.18637/jss.v035.i09

A BibTeX entry for LaTeX users is

  @Article{,
    title = {{JM}: An {R} Package for the Joint Modelling of Longitudinal and Time-to-Event Data},
    author = {Dimitris Rizopoulos},
    journal = {Journal of Statistical Software},
    year = {2010},
    volume = {35},
    number = {9},
    pages = {1--33},
    url = {https://doi.org/10.18637/jss.v035.i09},
  }