mitzählen
to count along
To count together with others or at the same time as a process.
Als die Lehrerin die Bonbons verteilte, haben alle Kinder laut mitgezählt.
When the teacher handed out the candies, all the children counted along out loud.

to count along / to count too
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to count along
To count together with others or at the same time as a process.
Als die Lehrerin die Bonbons verteilte, haben alle Kinder laut mitgezählt.
When the teacher handed out the candies, all the children counted along out loud.

'Mitzählen' (to count along / to count too) is a separable verb. The prefix 'mit-' goes at the end in a main clause. In subordinate clauses and in the infinitive with 'zu', the verb stays together.
Zähl bitte laut mit!
Please count along out loud!
The verb consists of the prefix 'mit-' and the verb 'zählen' (to count). The prefix 'mit-' expresses that you do something together with others or in parallel with something else.
Die Kinder zählten die Sekunden bis Mitternacht gemeinsam mit.
The children counted down the seconds to midnight together.
The literal meaning refers to counting numbers. The figurative meaning refers to relevance or importance and is often used in negative contexts to express that something does not count or is ignored.
Beim Versteckspiel muss man bis zwanzig mitzählen.
In hide-and-seek, you have to count along to twenty.
to apply / be valid
'gelten' and 'mitzählen' can both express that something is relevant to a situation. 'gelten für' means that a rule, statement, or condition applies to someone or something. 'mitzählen' means that something is taken into account in an evaluation, calculation, or decision. The words can be confused because both have to do with 'this also concerns ...'. The difference is: a rule applies to a person or thing; a point, result, or performance counts too.

Bei der Endnote zählt die mündliche Mitarbeit mit.
Oral participation counts toward the final grade.
Diese Regel gilt auch für neue Mitarbeiter.
This rule also applies to new employees.