People getting treatment join in wearing underwear (2-8)

'people' is the definition.
I know nothing about this answer so I cannot judge whether this works.

'treatment join in wearing underwear' is the wordplay.
'treatment join' becomes 'tie' (I can't justify this - if you can you should believe this answer much more).
'in' means one lot of letters goes inside another.
'wearing' becomes 'in' (in certain clothing).
'underwear' becomes 'pants' (underpants).
'in'+'pants'='inpants'
'tie' going into 'inpants' is 'IN-PATIENTS'.

'getting' acts as a link.

Can you help me to learn more?