I used to stop by the Delta compound when I was visiting Nha Trang to eat and drink,and socialize with friends I rarely got to see. This was back in 68-69,when it was still formally called Detachment B-52.
"Delta" was their code name that told "those in the know" their area of operations and gave a good hint as to what they did.
Even back then you didn't "apply" to Delta/B-52. You had to be invited,and nobody got invited that people already there in operational detachments like their "Road Runners" or their recon teams had not previously ran missions with them and was willing to vouch for them personally.
So,yeah,I guess that technically,you CAN volunteer,but if nobody there knows you and is willing to vouch for you,you are not getting in.
BTW,they probably had the best damn NCO club in VN. Excellent food,pretty waitresses,and top quality whisky at cheap prices.
NOT the place you wanted to go if you were looking to start trouble,though. Nobody in there knew how to back up,including the waitresses,who were watched over by a 6'8" tall Master Sgt who was a 3rd degree black belt in Shorin-Ryu Karate. I was friends with him and about half the other guys there from my time with the 1st on Okie.
His name was "Ed Clough",and he was an original Delta member that had a heart attack while running a platoon operation with them in the Delta in 64 or so,and had a heart attack after running across a rice paddy in the open a couple of times to throw wounded soldiers on his back and run back to to cover with them. It was hot like a MoFo in the Delta,and at 6'8" and 300 or so lbs and 50 years old,he was just too damn old to be running operations.
So they gave him another Silver Star and made him the 5th Group NCO Clubs manager.
Yeah,the waitresses were pretty safe.
BTW,in the spirit of full disclosure,I volunteered for Delta myself in 69,and was turned down because there was no one there at the time who had ever ran combat operations with me so they could vouch for me. Training didn't count. Had to be combat missions under fire.