What is the verb in this sentence?
The friends had to get the door open soon.
I think it is open, but I am not sure.
Yes, "had to" is considered the verb here. The verb "had" typically indicates possession, but in this case it is actually a phrase that's used instead of the modal verb "must." It is actually the past tense of "have to." So the sentence could be rewritten as, "The friends must get the door open soon."
"get" is also a verb, but it is a bare infinitive. The main verb "had to" (or "must") takes the infinitive "to get" as its complement.
Note: the sentence can be simplified by just saying, "The friends must open the door soon." (In this case, "friends" is the noun, "must" is the (modal) verb, "open" is the bare infinitive (i.e. "to open" without the "to"), "the door" is the direct object, and "soon" is an adverb.)
Had (past tense of 'to have').
Compare "The friends must eat" and "The friends eat". The action in the first case is clearly not eating.
It is even easier to come to the answer if the sentence is shortened to "Friends had to"....... HAD is the verb." "Open" I think would be an adjective as used in your question.