Engagement functions go by many names, roka, sagai, ring ceremony, nischitartham, and range from a small family gathering to a full-scale evening. The invitation should name the function the way your family does, make clear it is the engagement (not the wedding), and carry the essentials.
Use the families’ version for relatives, the couple’s version for friends, and the Hindi one for family groups. If the wedding date is already fixed, a single line announcing it (“wedding to follow on [Date]”) is welcome; if not, leave it out.