I’m level 9 now, almost to level 10. If you’re playing solo, it is hard/slow to level. If you join the G+ group for your faction in your area, and get hooked up with active local Hangout chats, it is a lot easier to level up, learn some of the local etiquette, and have more fun, too. It’s a blast to walk around a portal-dense area with a dozen players and smash & hack! On the other hand, if a hard core player starts making noises like the game is more important than real life, I’d advise shutting them down quickly. There are some sane, level-headed players in my area, and on my side of town, that won’t hesitate to back-up those of us that can’t be at every event.
As far as adding resos to portals that have already been claimed… if they’re in a high traffic area and already mid-level, probably safe, but if it is way off the beaten path, and has say one 8 and three 1s, it is probably because someone is trying to keep it from being noticed. (possible reasons include trying to get Guardian badge or saving it for a planned, coordinated operation)
Link Amps are mostly useless, unless a planned op includes a BAF (Big-ass field, like regional size), or you’re messing with someone (fill the Mod slots with useless mods… #HeExtends).
And yeah, portal level is sum of resos divided by 8.
Some more hints…
Heat sinks and Multi-hacks come in different rarity levels, which have different strengths of effect.
An unmodded portal will burn out for you if hacked 4 times in 4 hours, and once burned out, will take 4 hours from the time of the first hack to recover. This can be sidestepped by throwing on a heat sink - this will give the player that installed the HS another 4 hacks. This is helpful if you’ve gone out of the way to hack some keys and don’t want to wait f o r e v e r.
Throwing on a heat sink is also a way to immediately bypass the default 5 minute cool down period before you can hack again.
Those two are bonus effects… the intended purposes of Heat Sinks and Multi-hacks are as @e4tmyl33t stated.