
sich gesellen zu
to join
To join a person or a group of people; to come over and join someone.
Nach der Arbeit gesellte er sich zu seinen Freunden in die Bar.
After work, he joined his friends at the bar.
to join
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to join
To join a person or a group of people; to come over and join someone.
Nach der Arbeit gesellte er sich zu seinen Freunden in die Bar.
After work, he joined his friends at the bar.

birds of a feather
A proverb saying that people with similar interests, traits, or backgrounds tend to seek each other's company.
Alle im Buchclub sind Lehrer – Gleich und Gleich gesellt sich gern.
Everyone in the book club is a teacher—birds of a feather flock together.

to join / be added
To add something to something else or connect it with it. This usage is rather elevated or literary.
Zu den alten Eichen im Park haben sich nun junge Birken gesellt.
Young birches have now joined the old oaks in the park.
The verb 'gesellen' (to join) is related to the nouns 'der Geselle' (companion, fellow) and 'die Gesellschaft' (group, community, society). All the words have the basic idea of being together or joining others.
Er war ein freundlicher Geselle, der schnell Anschluss fand.
He was a friendly fellow who quickly found company.
The verb 'gesellen' (to join) belongs more to elevated or literary language. It is used less often in normal everyday speech.
Am Abend gesellte sich ein weiterer Gast zu unserer Runde.
In the evening, another guest joined our group.
In its most common meaning, 'gesellen' (to join) is used reflexively with 'sich' and the preposition 'zu' + dative: sich zu jemandem gesellen (to join someone).
Darf ich mich zu euch an den Tisch gesellen?
May I join you at the table?
The expression 'Gleich und Gleich gesellt sich gern' (birds of a feather flock together) is a fixed proverb. It is used to comment that people with similar interests or personalities often become friends.
Alle meine Freunde lieben Computerspiele. Tja, Gleich und Gleich gesellt sich gern.
All my friends love computer games. Well, birds of a feather flock together.
to join / get around to / be added
Sich zu jemandem gesellen and dazukommen can both express joining an existing group. Sich gesellen is used with zu and tends to emphasize deliberately seeking someone’s company. Dazukommen often highlights that someone joins the group or event later.

In der Pause gesellte sich Nora zu den Kollegen am Tisch, weil sie sich mit ihnen unterhalten wollte.
During the break, Nora joined her colleagues at the table because she wanted to talk to them.
Nora kam erst in der Pause zu den Kollegen am Tisch dazu, weil ihr Termin länger dauerte.
Nora did not join her colleagues at the table until the break because her appointment took longer.