。
We're expecting five guests tomorrow evening.
Literal
Tomorrow [possessive-の] evening five-person [possessive-の] guests [object-を] welcome [nominalizer-こと] [dative-に] become-is.
Built on the pattern ~ことになっている, which marks an established arrangement or schedule — 'it has been decided that X.' This is stronger than a personal intention; it implies the arrangement is set externally or by agreement. 迎える here means 'to welcome/receive' guests, not 'to pick up.' Notice the counter 五人 for people and the humble/neutral 客 for guests.