In a Slack, is there a way to see all the members that is part of that channel?

Another very quick way to get the users of the current channel is to use the /who command.

Just enter that command in the any channel and Slack will tell you which users are member of that channel. And the command and response will only be visible to you.

See here for a list of all build-in commands in Slack.

In the right top corner, to the left of the search bar, there is Show Channel Details switch, that shows and hides channed details sidebar, including members list.