Indeed, these are solitary bees (solitary in that they don't live in a communal hive/nest, even though females can dig individual nests fairly near each other). The vast majority of our native bees are both solitary and ground-nesting, though we can't see enough detail to determine exactly which bee this may be as hundreds of species live in MD. You may have a male and female in the photo; often, "swarms" of these ground-nesters are mating frenzies, combined with females scouting-out suitable burrow locations and then tending to digging and provisioning those burrows.

They may be specialists - around a quarter of our native bees are - since not all bees can make use of just any blooms. Often, they specialize in collecting the pollen of just one or a few native plant species, so they may be foraging outside of your yard to find them. Habitat for ground-nesters can be hard to come by with human development covering most bare ground with pavement, vigorous invasive plant species, or dense turfgrass, so it's beneficial that your site is useful to them.

They are non-aggressive and do not sting to defend a nest. They should not bother any person or pet, though they may fly close to you simply because you're blocking their flight path and they're trying to get by. We recommend leaving them to their activities, and within a few weeks or less, the adults will have died-off and their offspring will remain underground until next year.

Here are examples of commonly-seen solitary ground-nesting bees:

https://extension.umd.edu/resource/mining-bees-lawns

https://extension.umd.edu/resource/meet-pollinator-cellophane-bee

http://bugoftheweek.com/blog/2013/1/28/spring-beauties-plasterer-bees-and-yellow-faced-bees-icolletidaei?rq=bees

Miri

You're welcome.