After I join a group or event, how do I go back to swipe on more users?

Once you have selected an event or group under the "Search" tab on your navigation and selected "Join" on the ensuing modal, you'll be able to find that event under the "Swiping" tab, which is the icon on your navigation bar that looks like a deck of cards.  All the events and groups you have joined will be listed here, and, when selected, will show you the next Profile Card in the deck for that event.