conjunction reduction - How this present participle is formed

Can someone explain how the 'joining' part in this sentence is formed? and what can be the original sentence before reduction?

Joining us here in the studio to start things off we have expert Sonia Tarrington, from the University of California's Nutrition Research Centre.

I suppose the ing form at the beginning of the sentence cannot be considered a reduced adverb clause because the subjects cannot be the same.
Then I thought it might be a reduced adjective clause which relocated to the beginning of the sentence? Something like this maybe? But is it possible at all to relocate adj. clauses?

We have expert Sonia Tarrington, from the University of California's Nutrition Research Centre, who has joined us here in the studio to start things off.