Facebook – How to stop getting notifications about people I don’t follow posting statuses on Facebook

From Facebook's point of view, this is a feature, not a bug. FB wants group admins to deal with members ASAP. Your main options are:

(edit: open groups have the same approval issues as closed ones)

1. change it to a page so people can join without approval.
2. change it to a secret group so people can't request to join.
3. give up your admin role and have someone else deal with it.

See also: Any way to auto-approve requests to join new-style Facebook group?

Reply with “OFF” to the Facebook texts and it should stop. More info here on this Facebook support page.

If that doesn't work, I would contact Facebook support. They are not super responsive but they can solve problems. You could also block texts from the Facebook number. This is more of the nuclear option so try to work thru Fb first. More info on how to block numbers here.

For future reference, all I did was Google "someone used my phone number for Facebook" and the Facebook support article was the top result.

Edit: As per Al E's suggestion, you could try associating your phone number with your account. I might trigger a verification process to see who has control of that number and might fix the problem.