Unless you name specific Slack users when requesting the call, then the entire channel will be invited.
If you only want to invite certain people onto the call, it’s probably best to have a discrete discussion with Mobot about it first, otherwise the people whom you didn’t want to invite might feel left out.
If you only want to invite a couple of people onto a call open a private DM session with Mobot to avoid any confusion.
For example, DM Mobot something like:
“Set up a call between @jeff and @jill” and he will initiate that for you.
If you really want to invite everyone to the call, then ask Mobot to initiate the call right from the team channel.