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

1:1 chat

Chat group settings