I'm not sure about the lounge. Generally this is not possible, as you will need to show the boarding pass, however in FRA
there is a lounge specially for this effect, which is the Welcome Lounge, and maybe they have something similar in MUC
. You should check the Lounge Directory on the miles&more website.
You can't meet your friends in the international zone, as they will arrive from a Schengen flight and have no access to the international zone without a valid boarding pass. You may not go the Schengen airside zone as well, since after your arrival with SQ
you will go landside and then need to go back airside a level down to the Schengen area. This would be the normal way if you are having a Schengen connecting flight. But without the onward boarding pass they wont let you through and therefore you can't pick them up at their gate.
Therefore you will need to meet landside. The most logical would be arrivals area (it's a common one for Schengen and non Schengen). Just wait behind the sliding doors landside and they will come out sooner or later. Can't miss them. I'm quite sure there is a meeting point in the arrivals area, too.