مَدْعُوٌّ
Translation: One who is called / invited.
The underlying weak root contains wāw, and the passive-participle pattern contributes another wāw. The two combine through assimilation, yielding a doubled wāw in the written form.
FormPassive participle
واحد مذکرمَدْعُوٌّ
تثنیہ مذکرمَدْعُوَّانِ
جمع مذکرمَدْعُوُّونَ
واحد مؤنثمَدْعُوَّةٌ
تثنیہ مؤنثمَدْعُوَّتَانِ
جمع مؤنثمَدْعُوَّاتٌ
دَاعٍ denotes the caller; مَدْعُوٌّ the one called. The distinction is morphological as well as semantic.