The rule of thumb is that it is 30% easier to ride in the centre of a group. Solo I can average 15/16 depending on the route. In a group I can hold my own at 18/19 over 70/80 miles though we do usually average around 17 as that's where we feel happiest.
You should be able to find a club with a group run that suits whatever speed you are currently riding at. My number one tip is do NOT ride at the back. Newcomers always do this and struggle. Get in the middle of the pack around the 4/5/6 position to get the most benefit from group riding.
A newcomer to group riding should, with the permission of the group, spend enough time riding at the back to develop the skills and learn the group etiquette and signals. This might take just the early part of the run, or it might take two or three runs depending on the cycling experience of the new group member. It's not easy riding at the back, because every slowing down for corners or traffic means an effort to get back on. But it is safer for everybody than having an inexperienced rider in the middle of a bunch - better to get dropped than bringing down other riders.
Of course this also depends on the group - a club's slow group will probably have a looser structure and a no-drop rule, while the faster groups will have a tight formation and the only concession may be a re-group at the top of longer hills and a wait for mechanicals.