By becoming a member of an organisation you can let the organisation (and others) know that you're interested. There are no further consequences linked to becoming a member of an organisation. This means you don't need to pay anything to become a member. The organisation can, however, see your profile and send you a direct message. For example, to discover whether or not there are suitable opportunities for you (outside of the listed opportunities or events).
Go to the organisation or opportunity page
In the left column, below the logo, you'll find a "Join group" button
Press on the button and then on "OK"
You will have submitted a request to join the organisation
You'll receive a notification when the organisation has processed your request