I can understand this aggravation so I'll take my best guess at how to help, though I think Redblood^ has a good insight.
From what I understand, it was a jump in with no original intent as far as the plot is regarded, but under the original cast it was developing into something more outlined. If certain members have left and new ones have joined, you can try to preserve the integrity and carry it on as redblood suggested. For that I would make sure the old posters are not returning, maybe use their disappearances/deaths to influence the plot, and then steer the newcomers as best you can. Alternatively, you could always abandon the original plot entirely; I believe it would depend on what the plot is to determine how feasible / believable doing this is, but it can be achieved if you work it correctly. If the collection of newcomers seems to be little more than chaos, try to create a string of thought and see which follow you, if you don't mind those not actively following it to branch into their own segments. If they are disruptive in anyway, i'd politely ask them what their goals and intent is. ^^