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.

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 connect / to bandage
'verbinden' and 'gesellen' can both express that something or someone joins something else and is no longer completely separate. 'verbinden' is more neutral and much more common: it can refer to people, things, ideas, or qualities. 'sich gesellen zu' is used mainly for people when someone joins a person or group. It sounds somewhat more elevated or narrative than 'dazukommen' (to come along / join). 'gesellen' without 'sich' can also mean that something is added or connected, but it is rather literary. The words can be confused because both express closeness or joining. In everyday language, 'verbinden' is usually safer; 'sich gesellen zu' fits when someone joins a group.

Nach dem Training gesellte sich Tom zu seinen Freunden auf die Bank.
After training, Tom joined his friends on the bench.
Das gemeinsame Projekt verbindet die Schüler aus zwei Klassen.
The joint project brings together the students from two classes.