If you want to add multiple users to the group, build the input with a flow variable before adding it to the "Add multiple users to group" action.
Step 0, create a flow variable that will contain users data
Step 1, get catalog variables (list collector values)
Step 2, do a look up of user records, where sysID is one of Step 1
Step 3, for every record found in step 2, add the appropriate data (not sysID) to the flow variable
Step 4, use the data in the flow variable as input to the "Add multiple users to group" action
