Based on my small research, there are several options how to translate that.
bezpečnostní služba = a group of people (institution) = probably the closest translation
1 person = člen bezpečnostní služby
ochranka = a group of people (institution) guarding other people or buildings (objects)
1 person = člen ochranky (so if you see a member of security guard, you can say "To je člen ochranky." or just "To je ochranka." – similarly as you can say "To je policista." or "To je policie." (i.e. the institution))
strážce objektu = 1 person watching premises
ostraha = a group of people (institution) surveilling, watching premises
bodyguard
this word is used in Czech, the pronunciation is "bodygárd" or "bodygard"
it is used for one person, plural would be "bodyguardi" (pron. "bodygárdi")
security
this word is also used in Czech, the pronunciation is "sekurity" nebo "sekjúrity"
it is used for a group of people (institution) = security guard à one member may colloquially be called "sekuriťák"
Security guard
my Lingea dictionary says: "člen ochranky" či "bezpečnostní jednotka"
slovnik.seznam.cz says: "hlídač; člen bezpečnostní služby; ostraha (objektu)"