I'm also taking the presumption you want to stick to 800m-1500m on the track. The other thing you'll have to adjust to coming from the Bay Area is travel to meets. There are outdoor home meets at CU, Colorado State, Northern Colorado, Mines all within a hour's drive of Boulder. Possibly Metro State in Denver, too. For indoors, CU and Mines host meets, but the rest are going to be further away, like CU-Colorado Springs and Air Force Academy. Of course, these are all at altitude and for outdoors you can have persistent, strong winds at any given meet. If you stay for summer classes, there's a twice monthly all comers series on CU's track that can randomly draw good competition.
The notable clubs and groups around town are:
Run Boulder AC
Plus a handful of smaller adult fitness clubs and groups
Rojas has a primary focus on individuals and the high school private coaching market, plus the fees are high last I looked. Could be good coaching, but maybe shouldn't be expecting a regular group for training. Tinman has some middle distance guys, most notably Drew Hunter, but high standards to join. Roots also has high standards to join but apparently few if any 800/1500 athletes. Harriers are chiefly about cross country, road racing, and marathons. BTC does show up in results for meets held at CU, cannot tell if any of them were running 800m or 1500m there except for a workout or just for fun. As noted above, that group far more regularly shows up in race results for road races, cross country, and marathons. Run Boulder AC (going by their IG) does it all, including traveling to the west coast for track races. Ask around, though, there could be pertinent details I've missed.