If it is online, they could send private messages to each other.
Or they could be members of a group that you are not in. If it
is in person, they can simply make a look at each other that you
don't understand that means, "What an idiot!" that they
understand.
It happens. Also, in public groups, someone can talk "behind
your back" publicly by having a kind of 'code" where they are
making fun of someone in a way that the person doesn't
understand, but they do. The secret is to understand all of the
codes, but not let them know that you know the codes.* That
gives you insight.
