Since, -고 있어요 doesn’t have any irregular forms, the verb stem for the verb 놀다 (to play, hang out) remains 놀. If -는 중이에요 were to have been added, the verb stem of 놀다 would be changed to 노 instead of 놀 since verbs with ㄹ 받침 have this final ㄹ deleted before the -는 중이에요 construction is added.