Within how many blocks do I have to be for wheat to grow?

You may want to look at this page, specifically "Wheat Not Growing."

It looks like the only distance from (any) player that matters is the essentially the view distance since the area has to be loaded for the wheat to grow.

According to the Chunks Minecraft page, "In multiplayer mode, a grid with a default radius of 10 (for a total of 21x21 or 441 chunks) is loaded around each player and sent to the player by default, although this radius can be configured to be between 3 and 15, usually only lowered with a poor connection home server. These chunks may have activity (mobs spawning, trees growing, water flowing, dropped items disappearing etc.)"