Join and join mean different things when they say "participate" and "join". The most important difference between the two is that:

Joining refers to joining a group, club or organization or other groups or associations, emphasizing that "individuals join the collective"; We can also say: Join.

Joining means participating in an activity, which is equivalent to participating.

For example:

She joined a band/club. She joined a band/club.

The debate sharpened when she joined it.

When she joined the debate, it became heated.