My co-op character stays at sl100. I get plenty of summons. Got 30 sun medals yesterday while at SL100 from Nameless King, Twin Princes, Sister Friede, and Soul of Cinder. Not to mention all the DLC bosses you can face with people since the recomended level for Ashes of Ariandel is 90 and The Ring City is 100. The summon range for a SL100 char is 80-120. So anyone between those levels will be able to play with you. It's the perfect summon level for the fun bosses.