quote from Rutherford book Enemies:
"Satan is a spirit creature, therefore invisible to human eyes; and also angels are spirit creatures and invisible to human eyes" (p.34)
In Bible scriptures, "angels" have made appearances to humans. Gabriel appeared to Daniel; angels appeared to Abraham, Joshua, Ezekiel, Lot, and others. These "angels" looked like people. There is no mention of them being gigantic or having wings, or being transparent, or being polterguists. As a matter of fact, they didn't look any different from other people.
If you believe that Hebrews 13:2 is truthful, then some have entertained angels without knowing they were angels.
How does WT reconcile their stand of invisible creatures when the Bible scriptures declare that angels are visible?