Sets the entity's ID. This number is used in subsequent calls to pX_selectgroup, for enabling or disabling physical entities. You can assign each object an individual ID or set multiple objects to share a single ID number. However, you must not use 0 as GroupID, cause this would always disable the entity. Every new entity is assigned to group 1 by default. So you don't have to call pXent_setgroup unless you want better control of which objects are enabled or should collide with each other.
Collision checking is disabled for objects of the same group ID (except group 0 and 1). Thus if you want two objects to not have collision detection with each other, set them to the same group ID of 2 or higher.

1 if successful, 0 otherwise.

• groupID should be a power of two so individual groups can be enabled and disabled using simple addition (cf. example). Values 0 and 1 are reserved. 0 is reserved because it could never be enabled using a bitmask (it always remains 0). 1 is reserved as a convenience so that new objects are automatically part of a group, yet can also collide with each other.

• This function can also be called for non-physics entities to set the group ID of the entity.

Fast

Example:

// player collides with bot and forcefield, but bot can walk through the forcefield
pXent_setgroup( player, 2 );
pXent_setgroup( forcefield, 4 );
pXent_setgroup( patrolling_bot, 4 );

while (1) {
  if (player_inside_building) {
    pX_selectgroup( 1+2+4 ); // activate group 1+2+4
  } else {
    pX_selectgroup( 1+2 ); // disable group 4 cause player has left the building
  }
}

See also:

pXent_addforcelocal, pXent_settype, pX_selectgroup