who'll /huːl/, contraction of who will. This is a standard, common form.
The apostrophe marks the dropped wi; ll spells /l/ and oo the long vowel /uː/ as in too.
Who'll be at the meeting tomorrow?
Some dictionaries also allow who shall as an expansion.
For variety, dictionaries show the same one-syllable word differently: British entries transcribe who'll as /huːl/, while American entries often give the respelling hool with /hul/. Both point to the same smooth pronunciation, not two different forms.
Watch the spelling trap in the middle: the written ll spells just one sound, /l/, and the oo carries the long vowel /uː/. Nothing is silent here, but the double letters could mislead you into imagining a longer word.
Who'll is normally read as the contraction of who will, and whether an entry also allows who shall underneath, the pronunciation stays /huːl/ either way.
Who'll drive us to the airport? Read the contraction as one quick syllable, /huːl/, gliding straight from the long vowel into the /l/ without a pause.