Get the person all the way to the hospital (or into EMT's hand) and give as much information as possible to the professionals trying to save their life. Since this isn't about how to do that, but about the people involved, my answer will focus on that. Ignoring them may not be the strategy that gets the person in trouble the medical attention they need as soon as possible. While I do not have first hand experience dealing with people ODing on anything 'harder' than alcohol, I do have experience there and in general handling groups in high pressure situations.
I am not going into depth on any medical aspect of dealing with this situation and will be extremely vague about such, this is focused only on people/situation management.
Quick and decisive action is important when a life may be at risk, as soon as you are aware of an OD, identify who has taken charge of the situation. If that person isn't quickly identifiable, be that person. Giving people clear tasks and displaying some calm and confidence (even if you don't have it) will dispel panic and get people moving constructively towards help.
Teamwork and Roles:
Hopefully you are not the only one interested in doing the right thing and helping. If you are, you'll be wearing multiple of these hats, with the main one being The 'Medic'. All other roles support The 'Medic'. These aren't strict or exact, but more guidelines for how to effectively leverage a group of people trying to help. Fill/assign people these roles as they are listed in order of importance.
The most important role, you're sticking with the victim until they are receiving medical treatment and taking the lead on decisions toward that goal. If you're the only person helping, this is you. This role should be given to the helper with the most relevant experience, if nobody knows what to do, pick the least squeamish/closest to victim.
Supports the Medic; gets them anything they need, helps them manage the patient, takes over when they need a break. Rarely will there be a need for more that 1 Nurse, two people are more effective at giving medical help, but 3 is a crowd and will more likely be slowing things down.
Everybody else trying to help can be most effective by preventing the Medic/Nurse from having to deal with other people. This role is highly situational, but generally comes down to clearing area around the situation, making a path for the victim to be moved if/when they are being moved and intercepting people asking "Whats going on?" "Are they alright?" and other questions that the Medic/Nurse don't need to be distracted by. Being calm, polite and firm when interacting with others will minimize panic and assure others that the situation is under control. Don't give out more information than needed, but don't hide what's going on.
If the host of the event has not been notified about the situation, Crowd Control also going to be in charge of notifying them. Hosts have ultimate crowd control powers and should be helpful, nobody wants a death at their party.
To be blunt, in a situation where somebody is overdosing, you're likely to find a people around who are more interested in avoiding interacting with police/emergency personnel than with saving a life. Once you have a few helpers and the host knows whats going on, let the general populace of the event know that EMTS etc will be showing up and that they are free to clear out/move to the backyard/get out of the room where the OD is and any place in between that and the door. People are less likely to cause issues or impede medical help if they are physically not present when the pros show up. Don't delay calling EMTs, immediately after the call is made, let everybody know that the call has been made and the pros are on the way.