Ana,
I have gone on a few different beginner to intermediate level group rides.
I actually did not observe that there was any drafting going on at all. I do know that in the more advanced group rides there was plenty of drafting going on. (I'll likely never be fast enough for those groups anyway ) I'm assuming from your lack of experience that you are sticking with an intermed. or beginner level group ride at first?

If you have never ridden in a group before and have never drafted or been drafted off of, I would say it is indeed wise to get used to riding in groups first and yes do warn people to please allow space around you for a while til you get comfortable in group riding. If that means you have to straggle behind and be last in line then so be it.
I remember the first couple times i rode in a group I was quite nervous and tended to involuntarily touch on my brakes way more often than when I rode alone. It was very different to me than riding alone or with a friend or with my husband. Nervous light braking could cause accidents if people are drafting around you. People could get hurt.
Personally I would say don't draft or be drafted until you are feeling somewhat comfortable riding in a group and you know all the little rules.

A key safety factor in group riding is to be sure to clearly signal/announce your intentions before stopping, slowing, turning, etc. Also, you are usually expected to point to or announce a road or traffic hazard to the person behind you whose vision may be limited by you.
When the group is initially gathering for the ride, ask the leader about what is expected in terms of group etiquette. It's good anyway for the leader to occasionally go over such things with everyone, and they might welcome the opportunity to review it all with the whole group there.