In pursuit of a concerted action, more than one individual must be involved. These parties, together, then map out, acquire a general consensus amongst themselves, and finally put into action the scheme of plan they set out to accomplish. They possess combined intent to carry out whatever action it is they wish to achieve.
When a pair or individuals, in addition to this grouping, decide to commit such a concerted action, which involves the execution and perpetration of a "common tort" or shared civil wrong, they embody the role of "tortfeasors" or more specifically, "joint tort feasors
The situations that are included under the umbrella of such concerted actions can be put into 3 categories: "substantial assistance and knowing tortious conduct," "substantial assistance and separate tortious conduct," and "problem: concerted action." Each is self-explanatory, with the last being a situation in which both individuals proceed equally in an act, with full knowledge of the consequences that may arise from it.
It is wise to know such specifics attached to concerted action prior to deciding to become a party or even an accessory to any scheme. You may have not known that you possess just as much liability as the original person involved.