If the group warm-up is supervised or lead, then just follow their lead.
You'll probably learn some useful techniques. They're almost duty bound to be qualified if they're supervising/leading a group warm-up.
Otherwise, just gentle stretching, leading on to more vigorous stretching, leading on to jogging, gentle running, not forgetting the upper torso also, even the neck and shoulders, just gentle stretching head/shoulder rolling.
You only need to allow 10 -15 minutes for warm-up, but the group session may take longer. The supervisors will want to make sure everyone is warmed up sufficiently. Don't worry if you start to sweat a little in the warm-up, that's about where you want to be, as long as you have a minute or so to recover before the start.
And don't go shooting off too quickly at the start. Look for your pace as early as possible and relax into it.
About the 7.5 - 8k mark assess your reserve stamina and start pushing then if you feel you're up to it. Re-assess again about the 9k mark.