Hi @golfnutt82,
1. To check if the SharePoint group already exists you can use a filter with the Title with a GET request. If the response is empty your group doesn't exist yet.
_api/Web/SiteGroups/?$filter=Title eq '@{variables('SharePointGroupName')}'

2. To add a user you can use a POST request against the specific site group.
Make sure you use the correct claim of the user.
_api/web/sitegroups/GetByName('@{variables('SharePointGroupName')}')/Users
{
'__metadata': {
'type': 'SP.User'
},
'LoginName': '@{variables('UserClaim')}'
}

3. When the site already has unique permissions you can use the addroleassignment method with a POST request to assign permissions to your new or existing group.
In the example below I am using contribute permission. Also make sure you use the correct principalid of the SharePoint group.
_api/web/roleAssignments/addroleassignment(principalId=@{variables('Id')}, roledefId=1073741827)
