Hello and welcome to the forum :)
REI3, the platform, does not have an integrated chat feature at the moment. It is possible to build a chat application - but to my knowledge, nobody did this so far.
When I need to communicate with my colleagures, I use dedicated tools for that. Most apps, like REI3 Tasks, have features to make refering to things easy - like using task keys, which are a unique code for each task.