You can group up with people wherever you are in the galaxy, as long as you know the names of the people. To go questing together, you'll obviously all have to be on the same planet. Usually, it doesn't take much longer than a minute to gather the group even if you're all on other planets.
Concerning requests, if I understand what you're wanting to know, you just ask through the guild channel if anyone can help with certain quests. If a guild member's on the same planet as you and around the same level, chances are s/he can help you.
About level differences: just make sure no one in the group is 7 levels above the recommended level for a specific quest, since you don't get any experience points for it then.
Other things to keep in mind: in a balanced group, you should play the role that fits your class and skills. This seems straightforward, but solo gameplay can be quite different from group gameplay. Certain situations require you to adapt, however. For example, my Mercenary (DPS) often took up the role of a modest healer when no other healer was around. In the later stages of the game, a lot more is required from group play and such things will not be sufficient, but for lower levels, this can work.
Also, quests like Heroic +4's can be quite overwhelming, with lots of elites and strong mobs. A CC skill (crowd control skill, which renders a target harmless for some time) is very useful in these situations, always make sure there's at least one member with such a skill for hard quests.