Thanks for the replies. This is some mighty good feedback fellas.

I'm kinda thinking the goal here, is to find the simplest, most natural method that prevents player fights over missions. Also, I would think that finding a system that newer players to the server can grasp and adapt to instinctively with the least amount corrective input as possible would help the server the most.
Right now, it seems that an amalgam of the best of the ideas you guys came up with are:
1. Mark map from any distance, not to claim, but to indicate intention only. (Or should we specify a distance?).
2. Announce participation in side chat when inside 1000m from mission. (Good even number).
3. Mission becomes "claimed" and property of the player once a percentage of the AI are killed. (Maybe 50% for simplicity's sake?).
4. Players are required to be in main server group to take on missions. (Makes map markers visible to other players).
These, aren't final and open discussion is encouraged. Please feel free to post your opinions, feedback and additional ideas on this system.
I must say that it is my hope that this system, once perfected, will basically implement itself as much as possible, and thereby effectively reduce stress on players new to our system and server staff alike. I 'm not sure if we should insist on the "
exact" wording of the engagement announcement as much, as long as it doesn't directly cause
actual conflict on the server. I would think that a reminder in chat from the other players or staff would be effective and would naturally come if a player makes a habit of trying to exclusively "claim" a mission without meeting the requirements. I would think said player would soon fall into the proper practice of their own accord.
Again, the whole purpose of announcing missions is to keep the stress factor low in the server, provide a hassle-free environment as possible, and thereby fostering the growth and harmony of the community. Unfortunately, though, it seems that most of the server heartburn from the missions, has not been from from players actually doing the missions and causing fights outright, but ironically, from the friction induced by trying to enforce what may be an unfamiliar and unnatural system for players new to the server.