When you created your workspace, you had the choice of inviting individual members or syncing from your group.
If you go to the admin panel on your workspace you'll be able to change what you entered on setup and do both options simultaneously.
If you go to the group section and open up group you'll be able to change the group id and rank requirement. You'll also be able to sync your workspace members with the Roblox group to catch up on any changes.
If you go up a bit you'll see the General section and in that you can find members, if you open this up, you can add people and give them a role in the workspace.