I'm currently working on a ArcGIS ModelBuilder Project where I want to use "Add Join" to look through a list of species (Table 1: converted from Excel -> Make query table -> Copy rows (to get index)) and flag up the same ones from a different table (Table 2) I made that contains some of the species that's in the other table (Table 2) and give me its classification (which is associated with the specie in table 1).
For this job, "Add Join" seems perfect, however it seems to be malfunctioning.
First off, I can't just add Table 2 directly to "Add Join" as "layer name or table view" BUT if I run the module, add table to "add to display" I can select the one from table of content. I find this to be super weird! This makes my module have some problems as it will keep stopping on other tables that I throw at it, for example if I want to do a different area or whatever.
I have also attempted to use "Join Field" which is much less picky in terms of what types of tables you throw at it but It doesn't give my the classification of the species at all. The entire row is just empty.
I'm not sure if I managed to explain my problem properly, hence I added a picture. 
Best Answer
Add Join requires that the Target is a Layer or a Table View, not a Feature Class or Table. The difference is that Layers and Table Views exist only in ArcMap Desktop and support display and record selections while the other is what is stored on disk and shown in ArcCatalog that does not let you select records. Copy Rows only creates a Feature Class or Table (ArcCatalog), not a Layer or Table View (ArcMap Desktop).
Use either the Make Feature Layer tool or Make Table View tool on your Copy Rows output (table 1) prior to making that the target of the Add Join tool then use that as the primary target of the join. The Join table can be a feature class or table and does not have to be a layer or table view.