Example:

Code:
SetEntProp(client, Prop_Data, "m_CollisionGroup", flags);

Here are a list of flag values you can use (I found in another post and its usefull):

--------------------------------------------
FLAG  NAME                           NOTES
--------------------------------------------
 0     COLLISION_GROUP_NONE           
 1     COLLISION_GROUP_DEBRIS,             Collides with nothing but world and static stuff
 2     COLLISION_GROUP_DEBRIS_TRIGGER,     Same as debris, but hits triggers
 3     COLLISION_GROUP_INTERACTIVE_DEBRIS  Collides with everything except other interactive debris or debris
 4     COLLISION_GROUP_INTERACTIVE         Collides with everything except interactive debris or debris
 5     COLLISION_GROUP_PLAYER              This is the default behavior expected for most prop_physics
 6     COLLISION_GROUP_BREAKABLE_GLASS,
 7     COLLISION_GROUP_VEHICLE,
 8     COLLISION_GROUP_PLAYER_MOVEMENT   For HL2, same as Collision_Group_Player
 9     COLLISION_GROUP_NPC               Generic NPC group
10     COLLISION_GROUP_IN_VEHICLE        For any entity inside a vehicle
11     COLLISION_GROUP_WEAPON            For any weapons that need collision detection
12     COLLISION_GROUP_VEHICLE_CLIP      Vehicle clip brush to restrict vehicle movement
13     COLLISION_GROUP_PROJECTILE        Projectiles!
14     COLLISION_GROUP_DOOR_BLOCKER      Blocks entities not permitted to get near moving doors
15     COLLISION_GROUP_PASSABLE_DOOR     Doors that the player shouldn't collide with
16     COLLISION_GROUP_DISSOLVING        Things that are dissolving are in this group
17     COLLISION_GROUP_PUSHAWAY          Nonsolid on client and server, pushaway in player code
18     COLLISION_GROUP_NPC_ACTOR         Used so NPCs in scripts ignore the player.