Group identification and definition within multiple user, multiple access, and/or MIMO wireless communications. A group identification definition field may be transmitted to a number of receiving devices for use in interpreting current or subsequently received packets that include a group identification field (group ID). The group ID can serve a number of functions such as indicating those receiving devices for which the packet is intended, the identification of fields within the packet corresponding to the various devices, certain parameters (e.g., code type, code rate, modulation type, etc.) associated with such fields within the packet, etc. The group identification definition field may be updated or modified to allow for modification of the manner in which subsequent packets, including respective group IDs, are processed. One of a variety of events may direct the group identification definition field may be updated or modified.