Teammates may communicate with each other at any time, unless they leave the play area. If they leave the play area and return, they are no longer permitted to communicate with their team for the remainder of that match. See section 4.5 for more details on team communication.
This includes using the restroom. Note that if a player needs to use the restroom, then by all means let them go. The player may return and continue their match, however we then ask that the player no longer assist their teammates. Using the restroom should never be a concession. See MTR 4.5 for more information on team communication.