join, join in, join to, join up, join with

For most purposes, join is a transitive verb and takes a direct object without any preposition (e.g., join a club).

• What knot should we use to join the ends of these ropes?

Sometimes join is followed by such prepositions as in, up and with for emphasis.

• Sabrina’s friends urged her to join in extracurricular activities at her high school.
• After a few choruses, Ryan was brave enough to join in the singing.
• On this special day, I join with everyone present to wish the newlyweds every happiness.

The verb join up specifically means to become a member of an association, a movement or the military.

• The Okanagan Sailing Club is looking for new members. Do you want to join up?

Join to is used when one item is being joined to another.

• Mara knows how to join this piece of the quilt to the next.