Teammates may communicate with each other at any time.
“Any” really does mean “any”. Players can’t abuse this to waste time, but otherwise there is no limit to how this works. Players can communicate at any time.
A quick and definitely not complete list of such times:
– When deciding what to draft.
– When deciding to mulligan.
– At each step of casting a spell.
– Between turns.
– After untapping creatures but before untapping lands.
– The car ride home.