Event ID 4732 comes from the Audit Security Group Management subcategory and fires every time a new member lands in a security-enabled local group. Microsoft's documentation states that it generates on domain controllers, member servers and workstations, and that every added member produces its own 4732. Add three users to a group in one PowerShell call and you get three events, each carrying one Member SID.
The phrase "security-enabled local group" trips people up. It does not mean "a group on a workstation". In Active Directory terms it means a group with the Security type (not Distribution) and Domain local scope. On a domain controller that covers plenty of high-value objects: the built-in Administrators group in the Builtin container, Account Operators, Server Operators, Backup Operators, Print Operators, Remote Desktop Users, and any domain-local group your team created to delegate rights on file shares or GPOs. The sample XML on Microsoft's own page is exactly that case: DC01.contoso.local logging a user being added to AccountOperators.
On a member server or workstation the same event covers the SAM-managed local groups, above all BUILTIN\Administrators. So one event ID spans two very different scopes, and the Group Domain field is how you tell them apart: it reads Builtin for a built-in group, the computer name for a machine-local group, and the domain name for a domain-local group defined in AD.
Two smaller behaviours are worth knowing before you build a rule. First, Microsoft notes you will typically see a 4735 (security-enabled local group was changed) with no visible changes immediately before the 4732; that is normal and not a second modification. Second, 4732 does not fire when a member is removed. The removal counterpart is 4733, so a group that was quietly emptied and refilled produces a pair, not a single event.