Credits.
Then, at the end of a period (say a month) the alliance takes it credits and distributes it to it's members (after 10% tax!).
So, let's say Mr Anonymous is a mercenary, and for whatever reason MRD decides he needs some more cannon fodder for his upcoming war. (Events completely fictitious!) He gives the Mercenary alliance 1 credit and that mercenary then appears in the MRD Fraternity for 24 hours. This happens, say 20 times in a month between various people & alliances. Come the end of the month, 2 credits would be deducted for tax, and the remaining 18 distributed between the mercenaries team.
Thus increasing the people amount wanting to be in said alliance, and as such, increasing the difficulty of actually getting into it, and indeed the selection process taken by the leader.
If possible, at the end of the 24 hour period, a list of alliance battles fought should be presented to the person who hired the mercenary. The mercenary should also remain anonymous, his name should remain in the mercenaries alliance list, but a fake name ought to appear during combat and what not (when he is fighting for a client). The person who hired the mercenary shouldn't be made aware of the persons real identity either, only the leader and the mercenary should know the true identity.
Also, when not in a clients alliance, No Mans Land ought to be their homeland. Lastly, it would open some great RP opportunities... "In a dark alleyway, two robed exchange items silently. The taller of the two passes to the other some coins, while the other hands over a scrap of paper. "His demise is at hand, consider it done" They nod at each other and depart in separate directions, never revealing the face under the robes to the other." (Obviously better written).
Or indeed...
"Smashing through a throng of guards and henchmen, Mr Anonymous wades through with force, men fall before him. Heads rolls and blood drips from the ceiling, the carnage left in his wake is... unfathomable. His determination is like no other, arrows pierce his armor, axes fly past his head and his blood trickles from every orifice... and yet he continues onwards, as though possessed by some incredible power. Eventually, he comes to the end of the room, brining his axes squarely into the chest of the final guard he looks up at their leader. "If you will not pay... you will pay DEARLY!" The leader grimacing, takes up his sword and lunges at the mercenary, grinning Mr Anonymous brings around his axe parrying the leaders blow and kicks him in the chins, bringing him to his feet. "Your end is at hand" The axe comes thundering down into the head of the leader with such force that the head, neck and chest are ripped open, blood gushing from the massive impact, Mr Anonymous grins and walks off to seek a doctor."
OK.. too long... I got carried away... and that also needs cleaning up... but you see the AL possibilities no?
Might be a bit much, not sure...