This is based on personal experience, it may or may not happen to you.
You'll soon endup with high debt if this process continues.
Based on your question I'm getting that it happens every month, it's time your friends should own their credit cards (atleast FD based).
Once or twice or during sales its fine but not every month that too without you knowing how much each person spending on your own card.
At the end of the day, it's you who suffers most when things went sideways.
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Easy way to track is to collect the money as soon as purchase completed or atleast end of that day. If they say, will give at month end means you're taking high risk.
Use reminders or notepad or as suggested by someone Split-bill kind of apps. But I strongly suggest it should not be every month.
Again personal views.