hinzukommen
to be added
To be added to something already existing or to occur additionally.
Zu den Materialkosten kommen noch die Arbeitsstunden hinzu.
The working hours are added to the material costs.

to be added / to join
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to be added
To be added to something already existing or to occur additionally.
Zu den Materialkosten kommen noch die Arbeitsstunden hinzu.
The working hours are added to the material costs.

'Hinzukommen' (to be added / to join) is a separable verb. The prefix 'hinzu' comes at the end in a main clause. In a subordinate clause or infinitive, the verb stays together.
Zum Regen kam auch noch starker Wind hinzu.
Strong wind was also added to the rain.
You can often recognize the meaning from the subject. If a person is the subject, it means 'sich einer Gruppe anschließen' (to join a group). If a thing or an abstract concept is the subject, it means 'zusätzlich auftreten' (to occur additionally).
Später kam noch mein Bruder hinzu.
My brother joined us later.
In the meaning 'hinzugefügt werden' (to be added), the preposition 'zu' is often used to express what something is being added to. The structure is then: 'Zu [etwas] kommt [etwas Neues] hinzu.'
Zu den hohen Mieten kommen nun auch noch steigende Energiekosten hinzu.
Rising energy costs are now also being added to the high rents.
The formulation 'Hinzu kommt, dass ...' is a fixed expression used to introduce another argument. It always introduces a subordinate clause with 'dass', in which the verb comes at the end.
Die Wohnung ist sehr klein. Hinzu kommt, dass sie keine Küche hat.
The apartment is very small. Added to that is the fact that it has no kitchen.
also / besides
'außerdem' and 'hinzukommen' can both show that something additional is present. 'außerdem' is an adverb and connects information directly: you simply mention another point. 'hinzukommen' is a verb and describes something being added to an existing situation. In the phrase 'Hinzu kommt, dass ...' (added to that is the fact that ...), it can introduce another argument in a way similar to 'außerdem'. 'außerdem' is simpler and more general; 'hinzukommen' puts more emphasis on the situation being expanded by an additional point.

Nach dem Training kommt noch hinzu, dass ich für die Prüfung lernen muss.
After training, added to that is the fact that I have to study for the exam.
Für den Ausflug brauchen die Kinder Brot, Wasser und außerdem warme Jacken.
For the trip, the children need bread, water, and also warm jackets.