To join this military unit you need an invitation from the Commander
It was started in the connection with the Swe-Fin agreement that Jämtland&Härjedalen should be Finnish. This deal was not supported by some people, and the ones rebellious to this deal (who wanted to make their stand) joined Thulsa Doom.
Eventually the region was once again Swedish, much thanks to Thulsa Doom. Nowadays the Unit simply takes no direct orders from the defensive forces of Sweden in order to avoid such decisions again. If there is an order that is highly prioritized by the HQ, they will have to PM the commanders of the unit and then they will make up their mind about the battle (This is usually not necessary. We are on the same side most of the times). The soldiers are free to be a part of TSM if they so wish.
Thulsa Doom also gives soldiers of the MU, unable to support themselves, supplies during the join booms that plays out on their IRC channel each night. The requirement to receive these things are Swedish citizenship and to wear the official Thulsa Doom avatar.