Hi D.J,
I’m a recent ROTC grad who was selected for UPT in the reserves and will be in processing soon. Guard is a bit difficult since there is no formal board for ROTC cadets. Basically, you’ll apply off the street like any other applicant and if you get selected by a unit, request a conditional release from AFPC. Whether or not that release gets approved depends on the needs of the AF, but I’ve seen several cadets do it. So while difficult, it’s still possible.
For the reserves (which is what I’m doing and several other ROTC cadets/graduates), there is a formal board that meets in the fall of your 3rd year (or 4th if you’re on a 5 year plan). It’s basically an unsponsored board strictly for ROTC cadets which is a great deal imo. If selected, you’re basically a free agent to any unit (most of the time) and at that point, it’s up to you to find a reserve unit (not guard). If you do find a guard unit, that’s another release and it could risk your reserve slot (not entirely sure if this is true but best to my knowledge).
TLDR: Yes, you can obtain a guard/reserve slot through ROTC, but the process is much easier in the reserves than the guard.