To visit a group that you are a member of, click the Switch groups button, the second button from the right of the navigation bar. On mobile devices you will find this in the drop-down menu activated on the button at the top right of the page.
You will see a list of the groups that you are a member of, organised by type, from which you can choose the one that you want. The groups that you most-recently visited will be shown at the top.
You can also see a list of all the groups that you are a member of by selecting the drop-down menu on your name at the top right of the page, choosing the My user profile option and then the Group membership view. This page will also show you additional status information for your groups, for example if you are still awaiting activation as a member or if you have been suspended.