beitreten
to join
To become a member of an organization, association, party, or community. This verb is used with the dative.
Sie ist vor kurzem einem Umweltschutzverein beigetreten.
She recently joined an environmental protection association.

to join
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to join
To become a member of an organization, association, party, or community. This verb is used with the dative.
Sie ist vor kurzem einem Umweltschutzverein beigetreten.
She recently joined an environmental protection association.

'Beitreten' (to join) is a separable verb. The prefix 'bei-' stands at the end in a main clause, while in a subordinate clause it remains attached to the verb.
Trittst du dem Sportverein bei?
Are you joining the sports club?
This verb always requires the dative. You join 'einer Organisation' (an organization, dative), not 'eine Organisation' (accusative). This is a common mistake.
Er möchte einer politischen Partei beitreten.
He would like to join a political party.
The perfect tense of 'beitreten' (to join) is formed with the auxiliary verb 'sein' because it is derived from the motion verb 'treten' (to step).
Meine Schwester ist gestern der Gewerkschaft beigetreten.
My sister joined the trade union yesterday.
You typically join an organization, a club, or a community. Common nouns are: 'Verein' (association/club), 'Partei' (party), 'Club' (club), 'Gruppe' (group), 'Gewerkschaft' (trade union), or 'Organisation' (organization).
Viele Studenten treten zu Beginn ihres Studiums einer Hochschulgruppe bei.
Many students join a university group at the beginning of their studies.
to record / take in
'aufnehmen' and 'beitreten' can both have to do with membership in a group or organization, but the perspective is different. 'beitreten' describes the action of the person becoming a member: you join a club, a party, or a group. 'aufnehmen' describes the action of the group or institution: it admits a person as a member. Learners often confuse the words because the result is similar. The grammar is also important: 'Man tritt einem Verein bei' (you join a club), but 'ein Verein nimmt jemanden auf' (a club admits someone).

Am Wochenende tritt sie dem Sportverein bei.
She is joining the sports club this weekend.
Der Chor nimmt im Herbst neue Mitglieder auf.
The choir admits new members in the fall.