This is a good question, and unfortunately the answer is, "it depends". The Spanish letter d has different pronunciations depending on where it comes in a word.
Word-initially, it will generally have a sound closer to the English d, although pronounced with the tongue behind the teeth, rather than upon the upper alveolar ridge (on the hard palate).
Intervocalically (between vowels), the Spanish d is normally realized as /ð/, that is, the same sound you make when you say the word rather. In some dialects, it may be /Ø/ (elided/silent) if it comes before the last syllable.
Word-finally, the pronunciation will vary a bit more. The four most common pronunciations are /d/ (like word-initial), /t/ (also with tongue behind teeth), /θ/ (as in thistle), or /Ø/. This is heavily dependent on dialect.
Now you'd think the answer would be easy. dar has a word-initial d, so it should sound /d/. However, in Spanish, many words will often be pronounced together as if they were a single word, especially in fast speech. Prepositions, object pronouns, demonstratives, and articles are the most likely to be "joined" to a more significant word, generally a verb/noun/adjective.
I would normally have the following word groupings in speech:
- (nosotros) (nos reunimos) (para dar) (la bienvenida) (a la profesora)
Why is this significant? Because each group is pronounced as if a single word, that is, as if it read nosotros nosreunimos paradar labienvenida alaprofesora. In this case, you can see that the d is now positioned between two vowels, and would thus be pronounced /ð/.
On the other hand, let's say you trail off after saying para because you were deciding between saying dar la bienvenida and saludar. Notice how the grouping changes:
- (nosotros) (nos reunimos) (para...) (ehm...) (dar) (la bienvenida) (a la profesora)
Now dar is back to its own word group, and will get the word-initial pronunciation of /d/.
Nosotros nos reunimos para dar la bienvenida a la profesora. You could also omit the