I have very distinct memories of reading this series and was sure I remembered the titles but Google is failing me!
It was about a girl who befriends a pair of siblings at her new school and they let her in on their secret, which is that they've found a portal to another world in the countryside near their house. The portal comes out near a city and they've been (I think?) taking artefacts from earth through the portal to make themselves rich.
There was a supporting character who was a goth girl who was also travelling to the other world to learn magic and had a romance with a boy from the other world who might have been royalty. She also befriended a set of twins who were both very powerful mages.
At the end all the characters got trapped in the other world due to a terrorist attack and were forced to escape through another portal. The later books in the series were about their adventures lost in the multi verse, with particular focus on a world called 'the library' which had many portals to other worlds and was filled with books documenting explorers' adventures. I remember a subplot where one of them got conscripted into the military in another world and tried to make the best of the situation by arguing his way into a promotion by claiming to know how to make nuclear weapons.
I'm certain the titles all ended in -land or -lands (Borderland, Shadowland etc.) but Google is only turning up unrelated books. I'm in the UK and it might be that they were published under a different title in the US. This would have been late nineties at the earliest and mid 2000s at the latest.
Does anyone else remember this and know where I might find them? I'm pretty sure I still own them but my childhood books are all boxed up in a storage unit.
ETA: made a last-ditch attempt at Googling it and tried adding 'uk' to the end of my search and the cover I remembered popped up in the shopping results! It is
Rites of Passage by Rhiannon Lassiter. I'll leave this post up in case it's of interest to anyone else!