Open groups can be joined by all neighbors from the visible neighborhoods and are suitable for topics with open exchange, such as mother-child groups. Closed groups require approval from the moderator, e.g., housing communities. Posts are only visible to group members. The status of open groups can be changed afterwards, but not vice versa.
Any neighbor from the neighborhoods where the group is visible can join an open group. They are suitable for all topics where new members are welcome and open exchange is encouraged. A good example is a mother-child group in the neighborhood or a group for regular meet-ups.
You can only join a closed group with approval from the group moderator. So you submit a membership request, and the moderator decides whether to accept you or not.
A common example of this is a housing community that exchanges information in a closed group about topics that concern only the house and its residents.
Posts in all groups (open and closed) are only visible to members, which means neighbors must first join the group to be able to read the posts within the group.
Note: The status of an open group can be changed afterwards. The opposite is not possible.
How to join a group is explained in the article: How can I join an existing group?