Responsa for Eruvin 7:6

כֵּיצַד מִשְׁתַּתְּפִין בַּמָּבוֹי. מֵנִיחַ אֶת הֶחָבִית וְאוֹמֵר, הֲרֵי זוֹ לְכָל בְּנֵי מָבוֹי, וּמְזַכֶּה לָהֶן עַל יְדֵי בְנוֹ וּבִתּוֹ הַגְּדוֹלִים וְעַל יְדֵי עַבְדּוֹ וְשִׁפְחָתוֹ הָעִבְרִים וְעַל יְדֵי אִשְׁתּוֹ, אֲבָל אֵינוֹ מְזַכֶּה לֹא עַל יְדֵי בְנוֹ וּבִתּוֹ הַקְּטַנִּים וְלֹא עַל יְדֵי עַבְדּוֹ וְשִׁפְחָתוֹ הַכְּנַעֲנִים, מִפְּנֵי שֶׁיָּדָן כְּיָדוֹ:

How is partnership made in a mavui? He sets down the (food) jug, [his own, if he wishes, in which instance he must assign it to them (for if it belongs to the others, no assignment is necessary)], and he says: "This is for all the people of the mavui." And he assigns it to them through his grown son or daughter or through his Hebrew bondsman or bondswoman, or through his wife. [He says to them: "Receive this jug and acquire it on behalf of all the people of the mavui." And the receiver lifts the eruv in his hand a handbreadth from the ground, for so long as it is in his (the owner's) domain, no acquisition (for others) is effected, and he says: "I have acquired it for them."]; but he does not assign it through his Canaanite bondsman or bondswoman, for their hand is as his (the master's) hand [and assignment (to others) is not thereby effected.]

Teshuvot Maharam