The LaTeX command for in (∈) is \in (element of).
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Write \in inside math mode, that is between $…$ for inline math or in a \[ … \] or equation block for display math. It is part of core LaTeX math, so no extra package is required.
As a relation, \in gets a thick space on each side, and a long inline formula may break after it. With unicode-math under XeLaTeX or LuaLaTeX, typing ∈ in a formula gives the same symbol. Not to be confused with \epsilon (ϵ).
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\epsilon (ϵ)\varepsilon (ε): variant epsilon\subseteq\supseteq\subset\supset\subsetneq\sqsubseteq\sqsupseteq\notin\ni\vdash\models\asymp\doteq\triangleqRelation symbols in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \in in math mode; it needs no extra package. For example, $a \in b$.
None: \in is built into LaTeX, so no extra package is required.