It’s easy!
When you create a new Space you can make it public, private, secret or paid.

If you want to update the privacy settings on a Space, you can do so anytime by going to your Space > Manage > Space Details.
If it’s a public Space, the invite will show the content of the Space and then the member will have the option to “Join Us” at the top. If a member clicks to interact with the Space, a pop up will prompt them to join it.

Auto Join Spaces
If you have public Spaces, you can choose to auto join members to those Spaces by selecting "Auto Join". This will automatically add all current and future members of your Mighty Network to this Space. Members can leave a Space at any time.

If it’s a private Space, it will prompt the member to request access to join:

If it’s a secret Space, the Space will show to the member, after they accept the invite to it.
If it’s a paid Space, the member will see the plan page for the Space:

To learn more about these options check out this article.