I generally tip the Executive / Club Lounge attendants - especially in larger lounges, where there may be 2 - 3 attendants / servers. €2 or maybe 3 for two of us stopping in the lounge, snacking and having some wine poured and brought to us.

I generally do not tip the Concierge (a higher level than a lounge attendant / server) if they register me, check me out, welcome me or give me general information; they are paid to do that, that's their job.

If the Concierge makes a booking, spends some time giving me information and referrals, arranges a special service for me or otherwise treats me beyond the norm (e.g. a bottle of champers and some cocholate-covered strawberries in the room,) I do tip. The amount of the tip will be proportionate to what was done - a booking at a nearby restaurant that is pretty much empty for early Sunday brunch gets a significantly smaller tip than booking an upscale and popular restaurant that would not have any bookings for a month if I called them.