When I deal with alliance, often I experience a single person/AI make decisions, say declaring war without the consent from the rest of the alliance. Or inviting a new player/AI to a allancie is also to easy. 

Have some sort of vote system where the person/AI can vote yes or no for certain action would make it more democratic in a sense. Having example to declare war, you need minimum 55% (Just a example number) votes to declare war. From my point of view that would make the player feel they have more control over all other faction in the alliance, but also make single person less powerful of certain action.

I saw a more nuanced suggestion in another thread that would be worth considering: the major political parties of every faction in the alliance impact the cost of alliance decisions.

Let's say your alliance has three players. Two players have Militarists as their leading political party, while one has Scientists. The influence cost of going to war would be very low, while the influence cost of a peace treaty would be very high. This would make it much more difficult for alliance members to simply end a war that they're winning. The Scientists party would also reduce the cost of trading techs between alliance members.

This system does a lot of things- it makes political parties much more impactful in the game, and it makes hacking a viable option for severely disrupting alliances. If you hack a home planet and change their political party to Pacifists, it makes it that much harder for the entire alliance to continue a war.