[Lipepva]

Em OnPlayerConnect:

pawn Code:

new string[56];
    new PlayerName[128];
    GetPlayerName(playerid, PlayerName, sizeof(PlayerName));
    format(string, sizeof(string), "- %s entrou no servidor.", PlayerName);
   SendClientMessageToAll(0xFF0000AA, string);

Em OnPlayerDisconnect:

pawn Code:

new string[56];
    new PlayerName[128];
    GetPlayerName(playerid, PlayerName, sizeof(PlayerName));
    format(string, sizeof(string), "- %s saiu do servidor.", PlayerName);
   SendClientMessageToAll(0xFF0000AA, string);

EDIT#: Ops, mals aew postamos quase na mesma hora :/