Johanna, this is a typical case where "date scaffolding" will help you. Search the forums for "scaffold" and you'll find lots of examples.
Create a new table (and Excel spreadsheet is fine). This table needs one column (although you can have more, only one is mandatory). That column is "Date", and it contains a list of all dates for whatever range you desire (for this example, let's say all days in 2018 and 2019 - 730 rows).
Use this table as the left side of a join to your user ID data. I'd usually start with a LEFT join (not an INNER). Now you have a data set with a lot of dates with no data, and some rows with User ID data.
In your Tableau viz, use this Date column from the new table as your X-axis, NOT the "created date" column from your source data.
That should get you what you need.
Thank you for your great answer. I just have one small question, how do I do if I don't have a range of dates, but instead want to have a relative date span. I would like to avoid going back into to excel document to add more dates. I have one date with data for when a member id is created and then want to add more dates that show how many have converted over time (i have data for the date of converted as well).
Thank you again for your help.