Guests have fewer capabilities than team members, but there's still a lot they can do in channels. Team owners can set guest permissions for channels on desktop or web. To set guest permissions:

1 - Go to the team name, point to reveal, then select More options  > Manage team 

2 - Select Settings > Guest permissions. Check or uncheck the permissions you want to allow.

3 - Your guests can now collaborate with team members in channels.