Short answer. Yes for the arrangement, no for membership. Article 1804 lets a partner share his own stake with an outsider without asking anyone, but that outsider does not become a partner unless all the other partners agree. The outsider deals only with the partner who brought him in.
What the law says
Every partner may associate another person with him in his share, but the associate shall not be admitted into the partnership without the consent of all the other partners, even if the partner having an associate should be a manager.
Civil Code, Article 1804 — Sub-Partner (Associate). Read the full provision →
A partner may share his own slice
Article 1804 starts from a simple idea: what a partner owns of the partnership is his to share. The Code says every partner may associate another person with him in his share. This person is often called a sub-partner or associate. Your co-partner does not need to ask you, and does not need to ask anyone else, before letting an outsider participate in his portion of the profits and losses. Whether he is an ordinary partner or even a managing partner makes no difference to this freedom. What he is sharing is his own economic interest, not the partnership itself.
But the outsider does not become your partner
The freedom stops at the door of the partnership. The associate shall not be admitted into the partnership without the consent of all the other partners. That means the outsider gets no voice in management, no direct claim on partnership property, and no right to inspect the books or bind the firm. He is a partner of your partner, not a partner of the firm. Only a unanimous yes from all the other partners can turn him into a real member. This protects the personal, trust-based character of a partnership: you chose your partners, and no one can slip a stranger into that circle behind your back.
What this means in practice
If your co-partner has quietly taken in an associate, you generally cannot stop the private arrangement between the two of them, because Article 1804 permits it. What you can insist on is that the outsider stay outside the partnership's affairs. He cannot show up to meetings, demand accounts from you, or act in the firm's name. The associate looks only to your co-partner for his share of that partner's profits, and he shares in that partner's losses in the same way. The rule expressly binds even a managing partner, so a partner's management role gives him no extra power to force a stranger on the rest of you.