TLDR; Assuming there is no contract (written or verbal) then the fact that you put in roughly the same amount makes it slightly easier to say - Equal Stakes, Equal Share (20% each).

Longer version -

Ouch. The fact that you are posting it here suggests that there is some disagreement between the 5 of you.

Let me guess - you think you should get more as the founder and source of the idea. Person B says "but without me you would not have continued with development, so I should get the same as you". Person C says, "if it wasn't for me then you then you would not have managed to survive or be as profitable (tax expertise)". Person D says something like "well I tuned the algorithms and increased profits by x% so without me there would be no profits".

Personally I can't imagine investing 10k in to something without something in writing (in fact I'm truly trying to imagine the scenario and I just can’t). What was the agreement when they invested their money? Not trying to be snarky but I just can't see a situation where the conversation went "We need more money", Person B (or C or D) just went "cool, here's 10k, enjoy".