Short answer. Yes. Once you are sued under the group's common name, the rule requires that in the answer of such defendant, the names and addresses of the persons composing said entity must all be revealed — your answer genuinely has to disclose everyone who actually makes up the group.
What the law says
In the answer of such defendant, the names and addresses of the persons composing said entity must all be revealed
Rule 3, Section 15 — Entity without juridical personality as defendant. Read the full provision →
Why you can be sued under the group's common name at all
The rule addresses exactly the situation you describe: when two or more persons not organized as an entity with juridical personality enter into a transaction, they may be sued under the name by which they are generally or commonly known. An unregistered group without any separate legal personality of its own can still be named as a defendant collectively in a lawsuit, using whatever name it happens to be commonly or generally known by among the people it deals with.
The disclosure requirement in the answer
That convenience for the plaintiff comes with an obligation for the defendants. The rule states plainly that in the answer, the names and addresses of the persons composing said entity must all be revealed. The word must makes this a genuinely mandatory part of the answer, not something merely left to the defendants' own discretion to include or quietly withhold as they see fit. The rule does not carve out an exception for members who would rather stay unnamed, nor does it let the group answer collectively without also identifying who actually composes it.
Why the disclosure requirement exists
Suing under a common group name is a practical accommodation for a plaintiff who may not know each individual member's identity when filing the case. Requiring full disclosure in the answer closes that gap once the group responds, so the case can proceed against clearly identified, specific individuals rather than remaining directed indefinitely at an informal, vague collective name that names no one in particular. Any resulting judgment can then actually be enforced against real, identified people and their own property, instead of against a name that describes no one in the eyes of the law.
What this means for your answer
Because this requirement is stated as mandatory, your answer needs to list the names and addresses of everyone who makes up the group being sued, not just those of you who happen to be filing or drafting the response. Leaving even one member out of that disclosure would not fully satisfy what this specific provision actually requires of the answer being filed.