//the command
if ( myFaction == some_faction )
{
  //allow
}
else
{
  //tell them they aren't allowed etc
}

// Put this inside the command
if(gteam[playerid] == Team_TEAMNAME){ // If you have gTeams put the teamname here. 
// Action
}else{
SendClientMessage(playerid,0xDEEE20FF, "You are not authorized to use this command.");
return 1;
}

// Example
// TOP of your script
#define Team_Admin 0

// Under OnPlayerCmdText

if (strcmp(cmdtext, "/kill", true) == 0){
if(gteam[playerid] == Team_Admin){
SetPlayerHealth(playerid, 0);
}else{
SendClientMessage(playerid, 0xDEEE20FF, "You are not authorized to use this command");
return 1;
}} // Might have forgot one bracket ( }) so If It gives you an error when you try It, add another or remove one.