Tell them
Tell 'em
/ˈtɛl əm/ElisionLinking
What happens to the sounds
Unstressed them loses th and sounds like /əm/, attaching to the previous word: tell them → “TELL-em”.
When to use it
Same with get 'em, love 'em. Normal spoken English in any style.
Casual → formal
Slang
tell 'em
Neutral
tell them
Formal
inform them
Sound notes
Elision: a sound disappears completely (often t, v, h or th).