You simply follow the group!
I'd recommend first seeing if there is a group in your area by clicking "View Regional User Groups". To join one, simply 'Ask to join this group' and the leader will accept / decline regularly.
To see who leads our user groups, you can search 'People' and filter by 'Accomplishments' and 'User Group Leader' and see a list! To see what we look for in a leader, you can view User Group Leader Responsibilities & Benefits.
Remember, you can find all user group related information in Marketo User Groups (MUGs) which can be found in the Browse drop down menu (User Groups), or by search.
Hope this answers your questions!