FLT CREW OF C560 TAXIED OFF PAVED SURFACE AFTER EXITING RWY. THE ACTIVE RWY WAS CLOSED AS A RESULT OF THE RPTR'S ACFT'S PROX.

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AS WE TAXIED CLR OF RWY 26L AT GRR; WE DEPARTED THE TXWY INTO SNOW COVERED GRASS. THE ACFT WAS SHUT DOWN IN THE SNOW/GRASS AND LATER REMOVED VIA TOW TRUCKS. 5-8 INCHES OF SNOW HAD FALLEN AND WE HAD TO HOLD WHILE THE ARPT WAS CLOSED FOR SNOW REMOVAL. WE EXPECTED TO SEE SNOW ON TXWYS AND RAMPS. THERE WERE NUMEROUS RWY AND TXWY CLOSURES AND WE HAD BEEN ADVISED OF SUCH. THE ARPT OPS IN CHARGE OF SNOW REMOVAL SUGGESTED THAT THE BEST TAXI RTE TO OUR FBO WOULD BE TXWY M THEN TXWY D THEN TXWY R. THE WIND WAS APPROX 300 DEGS AT 20 KTS GUSTING TO 30 KTS. ANOTHER FACTOR WAS THAT AT LNDG; ALTHOUGH WE FOUND THE RWY TO BE NICELY PLOWED; APPROX 4500 FT DOWN RWY 26L ON OUR R WAS AN ACR X ERJ IN THE GRASS. WE HAD NO IDEA THAT IT WOULD BE THERE AND ALTHOUGH IT WAS NOT A SAFETY FACTOR AS FAR AS ITS DISTANCE TO THE RWY; IT WAS A BIG DISTR. A BE1900 COMMUTER FLT LANDED RIGHT BEFORE US. I WONDERED IF THIS ACFT WAS THAT 1900 AND THAT HE HAD TAXIED INTO THE GRASS AFTER LNDG. THE ERJ WAS POINTED N (AS IN TAXIING CLR) AND THE INTERIOR LIGHTS WERE ALL ON. THE THIRD AND FINAL FACTOR WAS THAT THE TXWY/RWY LIGHTS AND SIGNS WERE CONFUSING THERE AT TXWY A2 AND TXWY M. TXWY A2 IS A 'FORWARD HIGH SPD.' IT IS RATHER FILLETED AND THE BLUE TXWY LIGHTS SEEM TO START 'EARLY' AND THEN THERE ARE A BUNCH OF THEM -- ESPECIALLY WHEN ADDING IN TXWY M. MY RECOLLECTION IS SEEING A TXWY A2 SIGN SHORTLY AFTER PASSING THE ERJ. THIS WAS FOLLOWED BY SEEING BLUE TXWY LIGHTS. WE CONTINUED DOWN THE RWY AND WERE LOOKING FOR TXWY M. I EXPECTED IT TO BE PLOWED; BUT NOT NECESSARILY 'CLR' OF SNOW. (TXWY M BASICALLY HEADS YOU RIGHT TOWARDS THE TWR AND ROTATING BEACON.) I RECALL SEEING AN 'M/A2' SIGN. AT THIS POINT; WE THOUGHT WE WERE ON TXWY M SINCE WE COULD SEE PARALLEL BLUE TAXI LIGHTS IN FRONT OF US. WE TAXIED FORWARD AND WHEN WE DISCOVERED WE WERE NO LONGER ON THE PAVEMENT; WE SHUT DOWN THE ENGS. OUR PROX TO THE RWY NECESSITATED THAT RWY 26L BE CLOSED UNTIL ACFT WAS MOVED. ALTHOUGH WX CONDITIONS WERE NOT IDEAL; THIS ACFT WAS CREWED WITH 2 PROFESSIONAL AIRMEN. WE WERE NOT TIRED OR RUSHED; BUT WERE DISTR BY THE ERJ. I BELIEVE THIS SHOULD HAVE BEEN AVAILABLE INFO. THE SIGNS AND LIGHTS; ALONG WITH THE WX (SPECIFICALLY THE SNOW); LED US TO AGREE THAT WE WERE AT A POS THAT WE REALLY WEREN'T AT. THE ACFT INCURRED NO DAMAGE AND THE CREW/PAX WERE NOT INJURED.