Astragalus membranaceus is one of the 50 fundamental herbs used in traditional Chinese medicine and is categorised as adaptogen. It has been traditionally used to treat loss of strength, some chronic illnesses and to increase the overall vitality of the system. Astragalus is usually used with other herbs, such as ginseng, angelica, and licorice. 
There is limited evidence for using astragalus for any health condition. There is also lack of high-quality clinical trials. Some preliminary studies are suggesting health benefits of astragalus, alone and in combination with other herbs, especially for the immune system, heart and liver .
Immune system boosting properties of Astragalus extract
As mentioned before much of the research is focused on its immune stimulating polysaccharides and other active ingredients from the plant, useful in treating immune deficiency conditions. In vitro and in vivo studies suggest some immune stimulating effects . When astragalus extract was prescribed to patients with viral myocarditis enhanced OKT3, OKT4 and OKT4/OKT8 cell ratios were noted , indicating improved immune response. Astragalus has shown in vitro antibacterial activity against Shigella dysenteriae, Streptococcus hemolyticus, Diplococcus pneumoniae, and Staphylococcus aureus .
Despite the lack of clinical trials on astragalus, some human studies do indicate stimulation of activity of immune cells in clinical populations. Specific extracts of astragalus appear to have immunological activities in ex vivo settings. 
Does it really increase absorption of amino acids and vitamins?
AstraGin™, a natural compound composed of Astragalus and Panax Notoginseng is claimed to significantly improve the absorption of critical nutrients (such as amino acids, vitamins, glucosamine) and improve ATP production. On AstarGin official page they mention that over 12 different in-vivo and in-vitro studies are backing up their claims. Besides a few abstracts published on their sites, we were not able to find them (at least not online).
We managed to find some other studies that could at least in part support some of the claims mentioned. Findings of Yin et al.  study in animals indicates that Astragalus polysaccharide (active constituents of Astragalus membranaceus) may improve the digestive and absorptive function and regulate amino acid metabolism to beneficially increase the entry of dietary amino acids into the systemic circulation. 100 mg/kg of Astragalus polysaccharide seems to protect the mice from death on endotoxin intoxication and prevents the lowering of ATP levels . The latter study also revealed antioxidative potential of Astragalus polysaccharide . Astragalus polysaccharide also increases insulin action and hypoglycemic activity at least in part by enhancing the adaptive capacity of the endoplasmic reticulum, which can further promote insulin signal .
Side effects of Astragalus supplementation
Astragalus membranaceus appears to have a very low toxicity and is considered safe for most adults [2, 9]. Because astragalus is generally used in combination with other herbs, its possible side effects are not well-known. Astragalus may interact with medications that suppress the immune system and may also affect blood sugar levels and blood pressure. 
- Sinclair, Steven. “Chinese herbs: a clinical review of Astragalus, Ligusticum, and Schizandrae.” Alternative Medicine Review 3 (1998): 338-344.
- National Center for Complementary and Alternative Medicine (NCCAM) – Herbs at a glance. Retrieved 29. June 2013
- Cho, William Chi Shing, and Kwok Nam Leung. “In vitro and in vivo immunomodulating and immunorestorative effects of Astragalus membranaceus.” Journal of ethnopharmacology 113.1 (2007): 132-141.
- Huang, Z. Q., N. P. Qin, and W. Ye. “Effect of Astragalus membranaceus on T-lymphocyte subsets in patients with viral myocarditis.” Chinese journal of integrated traditional and Western medicine 15.6 (1995): 328.
- Miller, Alan L. “Botanical influences on cardiovascular disease.” Alternative medicine review: a journal of clinical therapeutic 3.6 (1998): 422.
- Yin, F. G., et al. “Dietary supplementation with Astragalus polysaccharide enhances ileal digestibilities and serum concentrations of amino acids in early weaned piglets.” Amino Acids 37.2 (2009): 263-270.
- Wang, L. X., and Z. W. Han. “The effect of Astragalus polysaccharide on endotoxin-induced toxicity in mice].” Yao xue xue bao= Acta pharmaceutica Sinica 27.1 (1992): 5.
- Mao, Xian-qing, et al. “Astragalus polysaccharide reduces hepatic endoplasmic reticulum stress and restores glucose homeostasis in a diabetic KKAy mouse model.” Acta Pharmacologica Sinica 28.12 (2007): 1947-1956.
- Block, Keith I., and Mark N. Mead. “Immune system effects of echinacea, ginseng, and astragalus: a review.” Integrative cancer therapies 2.3 (2003): 247-267.