Just like creating a community, adding members to an existing community can be done from Data Studio.
The steps are very simple. Go to data studio and follow the three simple steps listed below -
Step 1: Select the users you want to add to a community.
Apply a filter to get a list of users you want added or simply select the records manually.
Step 2: Add the selected users to the community
After you select the records, go to Admin Actions > Communities > Add members to community
Step 3: Select a community to add the records
Here, select the community you to want to add the selected records to.