I just played through the whole game without knowing this. It's either not mentioned or I somehow missed it (which I doubt). The way I learnt about it was trough looking through the Russian forums.

Once a mechmind is close enough to you, you can communicate with him using the "~"(console, tilde) key this will bring up a bullet point stile menu which allows you to issue several commands to a mech.

Among other things, you can rob them, ask them to follow you, tell them to back off and even convince them without having to kill them using this menu. Convincing like this is very expensive but you get a useful clan member instantly.