A group of three people and one dog hike up a green mountainside.
Some people are exercising outside.
'Three people' can also be described collectively as 'some people', a hike up a mountainside is a form of exercising and also indicates they are outside.
Some people make up a group and hiking a mountain is exercising outside.
A group of people is some people, a hike is exercising, and a mountainside is outside.