Yes, join is a Scrabble word!

Verb

1. Become part of; become a member of a group or organization

2. Cause to become joined or linked

3. Come into the company of

4. Make contact or come together

5. Be or become joined or united or linked

Noun

1. The shape or manner in which things come together and a connection is made

2. A set containing all and only the members of two or more given sets