Can Umbral Aniimo spawn in group spawns?

Bloom events count as well: Umbral and Radiant variants can both spawn during them. That gives you four places to look for an Umbral: mass surges, grooming spawns, wild surges set off by nurturing the ecosystem, and blooms.

Mass surges are the easiest to plan around because they run on a schedule. The Bonesky mass surge runs from Friday through Sunday. A surge also drops Sparkling Crystals, which you can keep farming for as long as your Aniipods hold out. Every catch in a surge is a chance at an Umbral and a chance at crystals, so stock up on pods before you start.

Grooming and Nurture are the spawns you start yourself. Grooming can trigger a special group spawn, and nurturing the ecosystem can set off a wild surge. An Umbral can turn up in either one, so you can go looking for one on any day, not only during a scheduled surge.