Answer:
'Join'
Step-by-step explanation:
'Joins' is the plural or a way to say 'He joins the game tomorrow!" or something else as shown.
We are talking about people (plural) and not one person ( eg. "John 'joins' the drama club!")
Look at it this way!
There are always people who 'joins' in with the cheering at football games.
OR
There are always people who 'join' in with the cheering at football games.
Clearly, the second one makes the most sense. The sentence flows, is concise and has a connection altogether!
hope this helped!!