You can teach your followers new spells via console commands on the PC version. To do so, you will first need a hexcode for the spell you want to add.
To find out the hexcode, open up the console (by hitting the ~ key) and type in:
help "SPELL NAME" 4
SPELL NAME is the name of the spell you want your follower to have. For example, if you want your follower to be able to summon a Frost Atronach, type:
help "conjure frost atronach" 4
This will return several hexcode values. The ones you're looking for start with the 4-letter string "SPEL".
In this specific case, there are 3 spells for Conjure Frost Atronach. I'm pretty sure (but not 100% sure) that the spells are listed from least to most powerful. The first hexcode listed for Conjure Frost Atronach is
To give your follower this spell, target the follower with your mouse cursor while the console is open and type:
And voila, your follower now has the ability to summon a Frost Atronach.