Wii Deca Sports has no simultaneous multiplayer. The only way to play with others is hot-seat (turn-based) — pass a single Wii Remote around.
Each of the 10 sports (badminton, curling, figure skating, etc.) supports 1–2 players alternating turns. For real-time multiplayer sports on Wii, try Wii Sports Resort, Mario & Sonic at the Olympic Games, or Wii Party.