Use COLLISION_GROUP_WEAPON, then it won't collide with players, while still colliding with other things.
Then to make +use work, do the following. This is TTT specific stuff, for the record, won't work in sandbox.
-- In the SENT script, server or shared
ENT.CanUseKey = true
function ENT:UseOverride(ply)
ply:ChatPrint("You used me!")
end
Of course everything related to Source's standard use stuff, like SetUseType, will not affect this. You should run whatever you want to happen when a player +uses the entity in the UseOverride function.