[lackmail]

you have to plus one participant then format the message also you have to put return 1; so it should look like:

PHP код:

new Participants;
CMD:arena(playerid, params[])
{
    new string[90];
    InArena[playerid] = true;
    Participants++;
    format(string,sizeof(string),"[ARENA]: %s(%d) has joined The Arena  [%i Participants]",GetName(playerid), playerid, Participants);
    SendClientMessageToAll(-1,string);
    return 1;
} 


edit: i dont think you should make arena Participants[MAX_PLAYERS] it should be a normal variable just new Participants because

Participants[MAX_PLAYERS] < one variable for each player so when they join each player variable will count the player itself not other players