Open a channel and choose Members, then Invite people from outside the team. The channel becomes members only: everyone in the team now stays a member, and others in the team can join it from the channel list.
From Members, add someone by their handle. Someone in the team is added at once; anyone else is sent an invitation and joins as a guest once they accept. Or create an invite link that anyone signed in to Magpie can use for seven days, up to 25 people.
A guest can read and post in that one channel and leave it whenever they like. They see nothing else of the team. Their channels are listed under Shared with you in Chat, whichever team they have selected.
This works on the web and in the mobile app. General, and channels in Personal, cannot have guests.