Keep in mind, this is just a 'suggestion' i'm making based on my own experience and from my own point of view. If anybody disagrees, well, you are free to either post it here or just PM me. So, let's begin.
Opening a group
Once the idea of opening your own group crosses your mind, ask yourself if you really want to dedicate your time and nerves to actually lead a group. Make sure you are doing it for the right reasons - to roleplay and ofcourse to have fun. Once you are sure you are going to open a group, you should know that you are the one responsable for the group's and group members' actions. You are not only 'the guy who started the group' you are also the guy to lead the group. You need to make a good example of a role model to the group and the rest of the server. Next step is to set your group goals: Do you want to open a group just to join one of the countless alliances on the server just so you can lead your men into war, or do you actually want to roleplay a legit group?
Group goals
So, the only right thing to choose is to lead a legit roleplay group. Once you have your group running, don't just go around talking in your CB radio and sittiing in your turf doing nothing. Make a good use of every moment spent online. In my experience, best roleplays are the ones done in my hood with my group only. Simply because i know i can always count on them roleplaying realistically (unlike huge amount of the players all around the server), and we always have fun doing so. It's better to have 5 good members than 15 rulebreakers.
History/Background story of the group
Almost every group has one, yet these are just complete bullshit in most cases. The history of the group should have a meaning, so should a player's personal history (the one players write when submitting an application to the group). Further development of a group should be based on the history of the group. Same goes for players doing an application to a group. Everyone writes the same character story, later on theres 0 character development present. You should put more effort in such things, it will make roleplay way better. Hold on to your character story, and make the character you roleplay be the same person you described in the application you submitted. Just as an example, when i first joined GSF, i introduced myself to the gang as Lil' J (Short of Little J). These days everyone calls me Big J. After months, i've developed from a young gangster to an OG and later on to the leader of GSF (you don't need to take so much time and so many steps in your character development, but i've had a lot of fun doing it).
Leading a group
I guess this may appear to be easy, but it really isn't. Many hard desicions to be made, kicking friends from the group, accepting people you don't like, and such. It takes time and as i said above, you need to lead by example. You have to show the group members how to behave and what to do. You have to control your members and keep them entertained, else they will probably leave.
Group relations roleplay
Like 30% of it is actual roleplay and the rest is either being friends or war because of personal beefs and hatred between individuals/groups of people. Roleplay should be way different than that. We're on a light roleplay server, but that doesn't mean we should be killing people on every step. Groups should have a good reason to start a war, and even if two groups are in war, doesn't mean those groups have to constantly attack one another without a real reason. Roleplay, have fun with each other, roleplay is about playing together, and not against each other.
Group members, characters, ranks
Mafias mostly use ranks they know nothing about, and the players with such ranks don't really know what they are supposed to do. Think of ranks for your group on your own, make sure every rank has it's duty and position in the roleplay and not just in the /groupmembers or the forum topic. Don't just copy paste everything from other group's topic or internet. Be unique and original, that will make your group interesting and different from the most of the others groups around the server. Make group rank actually worth something, do duties you are supposed to be doing with the given rank.
Group inactivity period(s)
Every group has them, nothing to stress about. It's on you if you will come back after the inactivity, or just close the group down the way most people do it these days. Keep in mind that the strongest groups around here are the ones that never gave up despite inactivity of their members. Groups such as Corleone and Gvardia.
This is all for now, i'll possibly edit this in future hopefully with suggestions of others who think they can add something to this. Once again, this is just my opinion, people may disagree but i believe i know what i'm talking about.