Unlike other guilds the Muster make their money off of labour, not the provision of goods. They are a large consortium of organised Human Resources. The core of the guild is almost always mercenary. They specialise in providing corps upon corps of trained men for an adequate cost. They serve to provide mercenaries to whoever can offer the highest amount.
The Muster is also the largest resource of unionised freeperson labour, providing skilled (and unskilled) human labour wherever it is required. The protection of these unions extends only as far as the members of the Muster. They serve to provide an elite service and are exclusive about who they hire.
However the Muster has been known to resort to "volunteered" labour (aka slavery by more liberal groups) when they find the buyer wants cheap, easy and unskilled labour. Although slavery is technically illegal in the Empire this does not prevent the muster from Shang-haing peasants, freemen, barbarians and the occasional noble (sometimes deliberately for a nominal fee). Their slaves will often receive 'therapy' to help them re-adjust to their new lives (and condition them to forget their old). This trade in practice has earned them the nickname of "Chainers".
The recorded history of the Muster goes back to the Second Republic. There had always been a need for mercenaries among the new colonies. The demand was high during the chaotic Diaspora. The chaos of the end of the Second Republic again saw a demand for soliders for hire. Mobile Strategic and Tactical Resources was a mercenary group founded at this time.
After the Fall the company evolved to become the Muster.