Someone added me to a group, but it isn't showing up in my account!
If you received an invite message about your new group, then you should follow the invite link in that message. On the invite acceptance page, you'll see an option to merge the invite into your existing Splitwise account.
If you did not receive an invite message at all, it may be because your friend misspelled your email address. Ask your friend to go to the group's settings page and double-check your email address/phone number, and make sure that it matches the address/number on your account settings page.