After receiving the Mesmetron from Grouse in Paradise Falls, one has the option of enslaving non-player characters other than the four VIPs required to complete the quest Strictly Business. For every slave that is captured and sent back to Paradise Falls, Grouse will pay out 250 caps, and will hand over a new slave collar to continue the "recruiting." He also sells Mesmetron power cells at the steep price of 200 caps per 10 rounds. It's possible to offset the high price if all 50 rounds he initially gives out are carefully utilized, since he also pays out caps for turning in the VIPs.
A list of known non-player characters that can be enslaved with the Mesmetron. This may not be all of them. Enslaving a Tenpenny Tower security guard prevents the feral ghoul invasion from ever ending, due to a glitch where the ghouls seek and never find the enslaved (removed from the game) guard.