You’re in a new situation:
Terminology: what is industry standard that does not belong in our glossary? Or other constraints (3rd-party product’s terminology)
You’re likely to be the person who finds discrepancies across teams or terms
Hopefully part of the glossary can be customer-facing so that you can show that it’s worthwhile to be spending your time on this
How you write the definitions in your glossary: write carefully so you know what knowledge to assume the audience has.
Sources/SMEs for your glossary: YOU and your fresh eyes as you respond to practically anything you have contact with in your first days.
Once you have a glossary made, use the glossary to control terminology (programmatically ideally, but also culturally). Get people to agree on terminology.
Tease out internal versus external names
Think about naming conventions for APIs
Write definitions carefully, keeping in mind the audience’s level of familiarity with technology
Aim for wide distribution: Sales, Marketing, Support, Professional or Managed Services, Dev, UX