Greetings from the new moderator.
You will see in the top right of each loop's image whether it is open or private. You can read the contents of open loops, but you cannot comment. To comment you have to join the loop by clicking on the join button. Whether you can join a loop or not may depend on the moderator, so you may join immediately, or have to wait for acceptance. Be aware that some loops may have a reason to not accept new members.
*If you have been a member for some time, have a look around and you will see that there is much that is new.