Use of the apostrophe when possession is not the issue

What would be the best way to write "Beginners group" or "Childrens group" in "I'd like to teach the Beginners Group"? Both feel as if they need an apostrophe before the final 's', but this is not possession: the group does not belong to the beginners or to the children, the group is composed of beginners or children.
Any ideas?

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