Questing is usually a good activity to do with a small group of friends as you can always make sure that everybody has the items he or she needs to continue. Quests typically give good XP and/or unlock new content so it's a good way to progress in the game at a similar rate.
If you want something to do with a single other friend, you can partner up for slayer tasks by right-clicking an enchanted gem. Note that if the slayer master assigns a task that the other partner cannot do, he or she will not receive the task (so the player with the lower slayer/combat levels should be the one to request the task).
If you want to make fast cash together in OldScape, your friends can thieve silk from the stall in Ardougne and trade it to you which you can sell to the silk trader for 1.4k+/inventory.
If you are playing RS3, there are also distractions and diversions that you can participate in together which give excellent XP such as Penguin Hide-and-Seek, Shooting Stars, and Sinkholes.
For me, RuneScape is all about planning your adventure and figuring out what your goals are. You can start big (like completing every quest or getting 99 in every skill), then break it down to the major milestones along the way (like Heroes Quest, Tears of Guthix or 30+ in every skill). Then figure out what achievement diaries, quests will make getting there faster. If you can figure out what skills/items you need to complete your next objective, it should be pretty simple to figure out what you want to do next.