1. A method for wireless communication, comprising: selecting, by a wireless device, a first subset from a first set of candidate multi-user groups of stations, the first subset being based at least in part on first-level grouping metrics associated with the first set of candidate multi-user groups; and determining second-level grouping metrics associated with a second set of candidate multi-user groups, the second set corresponding to candidate multi-user groups having a greater number of stations or channel vectors than the first set, and the second set being based at least in part on the first subset.