Rock climbing (in the US gyms, anyway) is such an easy way to meet new people.

You don’t even to find a group or friends before you go. Just go to the bouldering area and hang out during a popular time.

Most gyms have partner finder programs and designated social nights.

Every gym I’ve been a member of has also had a bring a friend program where you get to bring one new person for free periodically.

Online groups are also a good way to meet new friends. This is HN so a lot of people will turn their nose up at Facebook but it’s full of groups of people who go out and do things.