Learn English – “Has been joined”

The short answer is that your understanding is correct.

The simple past (in the case of your examples, was) is used to indicate a discrete event that occurred in the past and ended in the past.

The present perfect (has been in your examples) describes an event that occurred in the past and might continue to the present time.

Without more context or information concerning time, there is the potential for ambiguity:

She has flown to Europe.

It has been a wonderful party.

In the first sentence, we're referring to one or more distinct past occurrences. In the second, the speaker is most likely referring to an event that is still in progress.

Either would be correct in this context.

Generally, the present perfect simple tense describes what has been happened so far, while the present perfect continuous suggests that something is ongoing or unfinished. The distinction isn't strong enough in this question to make a big difference.

Looking at two possible responses to the question might highlight the differences better.

(present perfect simple) I've worked here five years and I've loved every moment of it!

(present perfect continuous) I've been working here five years, but it's only recently that I was transferred to this office.

Similarly, one could say:

(present perfect simple) He's worked three hours today [he might work more, or he might go home after this].

(present perfect continuous) He's been working the whole morning [and he's still not done with his work].