I think if someone wants to add you on glip (ringcentral) they should have to be accepted by the other party. Long story short someone has added me on glip who I have not given my information to and somehow they just magically appeared on my "people" tab on glip 1 day after a missed call to them and only leaving my office number as contact info.
This was happening to us as well, and we determined that it happened any time an incoming caller happened to also be a RingCentral user. I believe we fixed the issue by turning off the "Allow off-domain co-workers" setting under Administration - Company Settings. Hope that helps!