megenreg has been superseded by merlin - please visit the merlin pages here. The below is provided only for reference.
Mixed Effects GENeralised REGression models
The megenreg packages implement the methodology described in the paper:
Crowther MJ. Extended multivariate generalised linear and non-linear mixed effects models. arXiv:1710.02223.
You can get the preprint here, which I recommend you read before trying out the accompanying software. The software is being constantly developed and improved.