Hi @sanjay,
You should consider whether you need the guest data type at all, especially if every guest will be a user. If you don’t need it then use your api results to create user records instead (using the action I mentioned previously). Just add the fields you have in your guest data type to the user one instead.
Assuming you do need a Guest data type then create a guest record as you’re already doing then in the next step in the workflow, create a user record. You can have a guest field on the user data type to link the two data types and you can also have it the other way (user field on the Guest data type) or both ways. It just depends on how you want to search the data. There are lots of posts in the forum about optimising for search.