učlaniti

1. (učlaniti se u + acc.) to join; to become a member
   Prošle nedelje sam se učlanio u teretanu. — I joined a gym last week.
   Učlanila se u gradsku biblioteku. — She joined the city library.
2. (someone + u + acc.) to enroll; to sign up
   Učlanila je sina u fudbalski klub. — She signed her son up for a football club.

In the sense 'to join' it is used with se: učlaniti se u stranku.