You have to talk to Wisp and scan your amiibo. In case you don't know how to get Wisp, walk around your village and you'll get a text message telling you to come closer. You have to keep following the text messages until Wisp shows up and gives you a lamp. For me, he was right near the train station, but it may be random. Once you get the lamp, you take it home and put it on the ground (or a table) and interact with it. Wisp will pop out and you can then scan your amiibo card or figure and select "come and play" and the character will be at the campground.
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Did this update patch out the glitches? I couldn't seem to get into the river anymore, though I haven't done it a long time and may be out of practice. But, I think it's patched out.