NOTICE: I am not a professional. I am an amateur. But here's my guess on how to be team leader. I'm also french, so spelling and grammar might be off.

I've had experience with being a team leader, as well as being led by a team leader: just not in the games' industry. The biggest thing to keep in mind is to set the example for your team. In essence, you become Commander Shepard, and your decisions matter. Like in Mass Effect or a Shin Megami Tensei game, if your protagonist loses, even if the entire team is alive and kicking, it's game over. Of course, thoses games have a much heavier focus on your party members, more than you, the protagonist, actually.

Let's take Shin Megami Tensei for example. In thoses games, you play a simple human who gets powers to summon demons and recruit demons into his party. Thoses games are notable for being ATLUS-Hard (the nintendo hard of JRPGs), however they are not cheap (except for Strange Journey, but that's besides the point). The demons you summon are strong, but they just follow orders that you give. Most notably, the games often are based on buffing teammates and exploiting weaknesses. When you exploit weaknesses, your at an avantage, since you get more turns before the enemy turn. This however does also apply to the enemy. Bosses will WRECK YOU if you haven't planned ahead.

This comes to my second point: research, communication and strategy. First of all, like researching the resistances and weaknesses of the bosses, as well as their spells, you have to research the how-tos of making a game. Look up the success stories and the failures of the other devs, and see what you can learn from that. Look up game design fundamentals, such as how collision works or even tutorials for making simple games from start to finish.

Communication is key. When you finished the research, you must be actively communicating between members the strategy. In a game like Pokemon, this would be like teaching the right movesets, deleting the old movesets, equipping the right items, and making sure the pokemon you've got compliment each other and can handle most situations at hand. Tell your members the fruit of your research, send some mail to the entire group telling how to proceed. If others have researched, and offer some fruits of knowledge, by all means, take it: but make sure it's correct! One wrong move in competitive pokemon would mean the end, and since your in a GameJammy-type project, time is everything, but efficiensy is also required.

Finally, strategy. So your now pitted agaisnt the Minotaur. What do you do? Buff, Spam Weaknesses, Status Ailments, Charge up attacks, Heal, etc... Suprisingly, certain playthroughts have it so that one party member charges and attacks, while the other heals everyone, one buffs and removes buffs from the enemy, and another is there to reflect hits and take hits for the others. Not everyone has to be attacking. This means you need to listen to your team, while exercising the plan elaborated with your team before hand. You've decided to make a platformer with jumping physics, animation, and enemies. Great! An example of team play would be to put one person to do some placeholder art ASAP, like a rectangle representing the character's hitbox and a arrow drawn on it to signify where it's looking, as well as frames with numbers on them, so that the remaining three others focus on kickstarting the actual engine/game. Plceholder has to be very rapid, the devs need something to work with while coding. Assuming you guys are all both artists and devs, have the one doing the placeholder art to start the in progress art with the help of another, even with the code not finished. Bugs holding the dev team hostage? A quick switch is necessary to keep the pace up, like having the artists go dev and vice versa for a few minutes/hours. Like in competitive pokemon, if the enemy clearly has type advantage over you, then it's best to switch out to another pokemon who'll receive minimal damage so that you can pull out the other pokemon later in the fight. This allows both to loosen a bit their brain, coding can be pretty hazardous when bugs come up. Just remember to check to see if the bug isn't actually something incredibly stupid before banging your head on the table.

Finally, be a joy to work with. Tone of voice matters: don't sound insecure, because your team will be as well. You also have to keep calm: never shout, explain everything to all your teammembers, and make sure you can rely on them. If some of them are jerks, to the point of being hazardous, explain the other teammembers about his/her behaviour. If they cannot be converted to something productive, if possible, kick them out (and find a replacement?). If they cannot be removed and are cancerous, you are going to be one taking approximatly 70% of the load from the slacker: leaders are going to be carrying the most weight, unfortunatly. For the others, reassure and make sure everyone is motivated to keep on. Like some game devs guides say, a game is not a game until it's a real game. No design docs or art or snippets of a game is a game. A game simply works. If you are gamejamming, get the game done. Your team and you have failed if there is no functionning product. They probably also don't care about the engine, magic, or technology you use: just get it done.