In most markets, there are fixed fees known as commissions. For instance, with a retail broker in the stock market, you can expect every trade to cost you $7.00 as an example, it is $7.00 regardless of if you place a trade for $25 or $25,000. You will see that just opening the trade, with a smaller amount, will eat up all of your profits and a majority of your capital, but if you opened the trade with more capital through the investment group, then the $7.00 commission will be much less of a tax on your trade.
Basically, the only advantage is that the tax of commissions will be less if you have a larger account, if the commission is a fixed dollar value, which is not always true either.
regardless, at $25 per month, not many markets will be accessible. There is also the possible educational aspect of investing with a group of people, or it can simply be clashing ideals.