Table Calculations are not supported for Forecasting. In this case, I'm not sure that is what is desired either. I think what is being sought is the number of members per month in the future. We can use a count([Member id]) calculation on the Rows shelf and a Month (Open Date) calculation on the Columns shelf to create a view that supports a forecast. The forecast for this data shows 3 new members per month through the end of 2016.
That forecast of 3 per month could be utilized in a new data source or the existing data source could be modified to include it.
If the ability to do a forecast off of a Table Calculation is seen as a need however, please consider voting on this idea - https://community.tableau.com/ideas/2682