The LaTeX command for ni (∋) is \ni (contains as member).
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Write \ni 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, \ni 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.
$a \ni b$Open this example in the live preview: it renders as you type, with no compiler and no signup.
\subset\supset\subsetneq\sqsubseteq\sqsupseteq\in\notin\vdash\models\asymp\doteq\triangleq\lessapprox\gtrapproxRelation symbols in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \ni in math mode; it needs no extra package. For example, $a \ni b$.
None: \ni is built into LaTeX, so no extra package is required.