The Combination under-20 centre made an encouraging debut for Reds' third team last weekend and now steps up to the seconds, who continue their push for promotion from Somerset One with a trip to Midsomer Norton.
Reds team manager John Kimberley said: "We're over the moon to have Lewis on board. He impressed at training and excelled on the weekend. We think he can go a very long way."
Beer made his mark in the thirds alongside Craig Owen, who continued his comeback from injury with another encouraging display that has earned him a starting position alongside Simon Mortimer in tomorrow's home National Three South West clash against Exmouth (2.30pm).
Kimberley added: "Luke has a knee problem that is going to take some time to sort out. We're not expecting him to play again until January, but it's good to see Craig back in action."
Reds have also signed prop Louis Wilson, who has previously played for Clifton and Barton Hill.
The National Three South campaign, meanwhile, reaches its mid-point with Dings Crusaders hoping they can end a run of five successive defeats with the visit of fellow strugglers Hertford (2.30pm).
The Lockleaze side have won only twice this season and sit one point off the bottom, but could yet turn their season around.
Clifton are level on points with Cambridge side Shelford, who they visit tomorrow seeking to make it back-to-back wins for only the second time this season.
Match of the day in South West One West is the derby clash between leaders Avonmouth and third-placed Clevedon, who make the short trip up the M5 to Barracks Lane.
The home side are without prop Ben Souch, who injured a shoulder in the Combination Cup win at Cleve. Fellow front-rower Jamie Lloyd is also missing with an ankle injury.
In Western Counties North, Bristol Harlequins welcome back Mark Ralph and Lee Pearn at centre for the home clash with Centralians. Pearn has been out of action for five weeks, but he remains Quins' leading try scorer with 11 touchdowns in eight outings.
Gloucester Premier leaders Saracens bid to make it nine from nine when they visit Richians as second-placed Frampton Cotterell visit Old Bristolians, who are only one point further back.