Hi @frob
You can create Public Team to create a team where your members can join without team owner's approval.
To do it
- click Join or create a team
- click Create Team
- choose a option to create a team
- choose Public
and those Public team will be visible to the members of the organization when people click Join or create a team menu and they can join the team without approval if they want to
If you want to change the type of the existing team from private to public, you can do so.
- click ... menu next to team name
- click Edit team
- change Privacy to public and click Done button.
Hope it will answer your question.
Thank you