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
Where 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 000204C4
.
To give your follower this spell, target the follower with your mouse cursor while the console is open and type:
addspell 000204C4
And voila, your follower now has the ability to summon a Frost Atronach.