can you provide a sample workbook or your geo files?
As @ZZ noted a sample workbook or the data would be most helpful. Going into your question there are a few ways to interpret what you have and what you want. Here are
1) I'm guessing you have two data sources, the members and the branches?
2) You stated you want to plot members by zip code. Are you plotting branches by zip code or lat/long? I'm asking because if you're using zip code and can use Tableau's built-in geocoding then you wouldn't need the custom geocoding, if you have lat/long then using the custom geocoding is better if you are using the dual axis approach (see below for more on that).
3) Where you said you wanted to look at proximity, is that just a visual proximity on a map or are you wanting to do some sort of distance calculation and/or generate trade areas for each branch? If you're just using visual proximity then you can do it all in Tableau, if you're looking for distance calculations or trade areas then you'll likely need to do some (or a lot) of pre-processing outside of Tableau to set up the data because Tableau's built-in geocoding doesn't expose the latitude/longitude pairs for distance calculations. (There's a way we can manually get at the lat/long pairs but not dynamically).
4) What version of Tableau are you using? The reason why I ask this is because Tableau's capabilities have significantly changed in the last few releases (though still aren't at the level of a GIS system).
Fundamentally you've got two options for displaying the branches:
a) Put them into an underlying map layer. This requires work outside of Tableau and hacking TMS files or using an external mapping service like Mapbox.
b) Use a dual-axis map where one axis is the branches and the other is the customers. In this case using custom geocoding gets you the lat/long positions, which is great, but to get two different sets of data (members and branches) onto the same map you currently have to get both 1) into a single data source where 2) all of the geographies come from the same source (i.e. the source if using a join, or just the primary if using a data blend). Sometimes we can get the single source via a regular inner/left/right join, however it's more common that we need a union. Tableau's union capabilities have expanded in the last several releases, however we still can't natively union completely disparate files (like a SQL Server source of members and an Excel file of branches). I put together http://drawingwithnumbers.artisart.org/cross-data-source-joins-join-on-calcs-union-almost-anything-in-tableau-v10-2/ to show how we can use Tableau v10.2's join calculations to do a union across sources. If you can edit the sources to add the equivalent join calculations as a column in the source (for example by using Custom SQL) then you can get the equivalent in v10.0 on up.