I want to talk to my mum but I can't get hold of her (this suggests she is not answering her phone or something similar).
I'm trying to get hold of someone at Microsoft. I've emailed and phoned but he just never gets back to me.
Could you please get hold of Dave for me? I need to talk to him urgently but I'm too busy to do it myself.
Be aware that "get hold of" can mean physical contact too, but not in the context you posted. In your post, Mel is saying that he (she?) needs the coach to contact Rana and Maddy. It appears the coach has already done so because he already knows they are due to arrive in 30 minutes. However, the coach will still need to contact them again to tell them to go to the boiler room.