Strategic suggestions:

Have one of these guys dancing in some forest clearing, moonlit area; some nice place for it to be found. It'll twirl and wind about, maybe crooning, and acts completely unhostile. Once they are hypnotized the second comes and nuzzles their legs, making sure to tap them with their antennae. Again, acting completely unhostile, but cute and playful.

Once the whole group is Charmed it may wrap about one of the players, tickling them with its fur, and they'll think nothing of it. Once the player collapses from damage they could manifest Modify Memory to make the other players think the guy simply lay down to rest.

Or maybe they'll make one think that another of the players struck him for no reason whatsoever. Something along those lines.

Half the group should be down before they know what is happening, and even those that are left are still Charmed.