Basically each day is split into 2 event times, noon and evening. Each of your friends can "occupy" 1 of the 5 girls during these times which means you can't talk to her during that event time. If you've never seen any of their scenes yet they will always be on the beach. As you progress through the friend events (you have to actually watch the friend events for them to progress), when they occupy 1 of the girls they will take them to the Red Light District (usually the bar or the love hotel, from my experience).
Afaik it should be random which girl each friend picks. If you see them with the same girl constantly it might just be bad RNG. Sometimes they even take a break and aren't talking to any of the girls.
It also has no gameplay affect on the girls. As others have said, you might see some NTR lines (if you can even understand the dialogue) but otherwise you can still have sex with all of the girls. You just have to wait until there is an event time when they are unoccupied.
And the friend events don't progress if you don't watch them yourself. You can think of it as your friends striking out if you're not watching.