While Dark Souls can be played with friends, keep it mind its co-op features are more intended as random help than full-on multiplayer. The biggest reason for that is that progress is not shared between the host and phantoms.
The host must be in human form to summon phantoms, and the summon range is based on your Soul Memory range.
The cat in Majula sells the Name Engraved Ring. When you equip it you choose a god and you're much more likely to match with people who chose the same god. It roughly doubles the summon range, but not the invasion range.
1- Use the small white soapstone. You get it from a chest above the Cardinal Tower bonfire in Forest of the Fallen Giants. The host can use small signs even if the zone's boss is dead. The summoned player will stay for about 10 minutes, but every kill shortens the timer. They can help with some weaker(?) bosses too if you're close enough.
2- Use the (regular) white soapstone. You get it from Pate after clearing the area next to him in the Forest of the Fallen Giants. You can only see "regular" signs when the zone's boss is still alive. A phantom summoned that way will stay until the boss is killed, or until you timeout (though you usually have enough time to clear the whole zone just fine).
I'm not aware of any official information about cooldowns and timeouts but unless your phantom dies immediately after summoning it, you should be able to resummon it just fine. For example, I summoned a friend through a small sign at the start of the Shrine of Amana and played until he fulfilled his duty, then went back a bit to a safe spot and could summon him right back. Same deal with the regular sign. I summoned a friend to kill the Skelleton Lords and by the time I got to the Harvest Valley I could summon him again.