pridruživati

1. (pridruživati se + dat.) to join
   Sve više ljudi se pridružuje protestu. — More and more people are joining the protest.
   Leti nam se često pridružuju rođaci iz Novog Sada. — In summer our relatives from Novi Sad often join us.
2. (something to something) to attach; to add
   Nove članove pridružujemo postojećim timovima. — We add new members to the existing teams.

The perfective is pridružiti; 'to join someone' is pridruživati se + dat. (pridružujem vam se).