Its important to be higher lvl as well. For clan vs NPC fief battles being lower than 27-28 can make it hard to get excepted to the NPC side.
Also check the merc forum in the strat area for upcoming battles. Its also important to make sure you jump into the Team Speak server, tactics are really important in strat.
And Knute is dead right, leave you cash count at 0 untill you get some rep, and always have it at 0 for NPC fief defence.
One last thing, you can only join with your main.