There are two different types of groups that you can be part of - Feed and Chat
Feed Group - In a Feed group, users can have post-specific threaded discussions with likes and comments.
Chat Group - A chat group is an Instant Messaging group where users can send text and multimedia messages.
Every group will have either of the three levels of access - Open, Closed, and Private
Open Access - The group is visible to all users, and any user can join instantly
Closed Access - The group is visible to all users but requires admin approval to join
Private Access - The group is hidden and requires an invite from the admin to join
To know more, check out the links below:
Join feed group
Post in a feed group
Feed group settings
Join chat group
Post in a chat group
Chat group settings